The problem with brokers is that they can charge 1-2 months rent. That means you can easily be left with a 8k bill for moving into a new apartment.
The job literally could not exist anywhere else because no one would pay it. But they aren't making any more land in Manhattan and they aren't building subway lines quick enough so landlords get to pass that expense off to tenants
If you are a top earning investment banker or lawyer, it's still worth it to pay that rather than spending 10+ hours of your time going to apartments etc. Most of "seeing apartments" is "let me get to the apartment by cab (stuck in traffic) or by subway (oh, delays, oh well)" so it's even less efficient for you to do that vs a broker.
The job literally could not exist anywhere else because no one would pay it. But they aren't making any more land in Manhattan and they aren't building subway lines quick enough so landlords get to pass that expense off to tenants