Orthodox Judaism has a huge amount of rules that are not directly found in the Torah or the rest of the Hebrew Bible (ie. "Old Testament").
As an example: In Orthodox Judaism you must make absolutely certain that you never, ever mix meat and milk. Not even a single molecule must be mixed. The strongest adherents of Judaism go so far as having divided their kitchen into two parts, where nothing crosses the line from one part to the other: Cookware, utensils, dishes, raw ingredients... Each half has its own set of them, and they must never, ever cross to the other side. Even the not-so-strong adherents will have at least some measures to make sure that no molecule of milk ever touches a molecule of meat by, at a minimum, never using the two ingredients at the same time and judiciously washing all cookware, utensils and dishes in between (although most prefer to just have two separate sets of them.)
Nothing like this is mandated in the Hebrew Bible, though (nor are there any mentions or examples of people doing anything like this). In fact, there is no commandment against eating meat and milk at the same time. Not even goat meat and goat milk at the same time. The only thing that's prohibited is boiling goat in its mother's milk. (This is a rather strange commandment, and some have hypothesized that it's actually metaphorical, and the people of the time would have understood what it actually meant. However, that's not important in this blog post.)
So if the prohibition is so specific, why go to such lengths and create a huge amount of rules that seem completely overblown and extreme?
The basic idea is that all these additional rules and mandates are there to protect a person from accidentally breaking a commandment. Rabbis themselves call this "building a fence around the commandments": In other words, follow rules that keep you as far as possible from even accidentally breaking a commandment. After all, if you have milk and meat in the same dinner, there is the remote possibility that the meat happens to contain goat and the milk happens to be from its mother, and if the meat is hot, it may end up being "boiled" in that milk, no matter how briefly. No matter how extraordinarily remote that possibility may be, it's better to err on the side of caution: Don't let even the remote chance become reality!
In other words, all these extra rules have the principle of "it's better to keep as far away from the line (of breaking a commandment) as possible, than to take any chances." No measure is too extreme to protect you from accidentally crossing that line. If using separate kitchen utensils protects you from even accidental breaking of the law, it's well worth it.
And, of course, this is only one example of literally thousands.
Naturally what can and can't be done has sparked endless debates among Jewish scholars and Rabbis over the centuries.
One of the topics that has sparked probably the most amount of discussion is what counts as "work", which is forbidden from being done on the Sabbath?
Obviously doing anything related to your job is clearly "work". But can something else also be considered such? What about doing chores at home, such as repairing something that's broken? Is that "work" that's forbidden during Sabbath? Again: Better to err on the side of caution and consider it such. Thus, you cannot repair anything during the Sabbath.
This "protective fence" around the prohibition of doing "work" during the Sabbath can sometimes go to rather ridiculous extremes. For example, a very common interpretation is that just switching on a light is "work", and thus shouldn't be done during a Sabbath. (The train of thought is that historically lighting up something required starting a fire, which can be quite exerting and requires tools and skill. Clearly it's work! Not so much different than eg. repairing something. Thus lighting a fire during Sabbath is best avoided. The modern equivalent is flipping a light switch to turn on the lights: Same thing, just a more modern version. Thus, once again, better to err on the side of caution: You shouldn't be switching lights during Sabbath.)
Carrying heavy loads, even if it's just inside your home, is also quite clearly considered work, as it's physically exerting, and it goes against the principle of the Sabbath being a day of rest.
But what about carrying very lightweight things? Can you carry your keys in a pocket, for example? Is that work or not? What if you have to carry something lightweight due to necessity, such as bringing food to a sick family member? Is that work?
After much deliberation the scholars and Rabbis came to the decision that carrying relatively light weights is allowed during the Sabbath, but only in your home. You must not carry anything further than that.
Of course the next question became: What exactly constitutes "your home"? Is it just the space confined inside walls? What about a patio, or a garden? Are you allowed to exit the door of your house carrying something?
A concession was made: Your property is your home, and that can include not just the house but also the immediately land around it, such as a patio or a garden.
However, in order to know where exactly the line is, it was decided that every Orthodox Jew who owns a home with property around it should clearly mark where the line is that must not be crossed while carrying something. This is marked with a so-called "eruv", which is a wire or string for this exact purpose: It marks the limits of the area where you can carry light loads, and must not be crossed.
And this is where the "abuse" of this allowance started happening. For example, if the properties of two neighbors were side-by-side, some of them would use a joint "eruv" wire that went around both properties, allowing them to carry stuff from home home to the other. After all, they didn't need to cross the line because the line was surrounding the joint property.
What if a third neighbor wanted to join the party? Well, extend the "eruv" to cover his property as well! Now all three could travel to any of the other two homes, carrying stuff, as no wire was being crossed.
And, thus, we get to the absolutely ridiculous end point of all this gameplay: The "Manhattan eruv wire". Which is an "eruv" wire that surrounds a good chunk of Manhattan. An absolutely humongous area.
Orthodox Jews who live within this area take this to mean that they can carry stuff within that huge area without worry. Which, rather obviously, goes against the original idea of limiting this area as much as possible.
In light of the other stuff about the "kosher" rules, which are designed to keep the faithful as protected as possible from breaking any commandments, going to absolute extremes to make sure of that, this ridiculous "Manhattan eruv" is certainly going to an absolute extreme... in the opposite direction! In other words, towards the line of what constitutes breaking the Sabbath and what doesn't. It's the exact opposite of the idea of keeping oneself as far as possible from breaching that line.
Which is why it just doesn't make any sense.