I think Chinatown is used as a symbol for helplessness.
Jake and Escobar used to be on the police force together in Chinatown, where Jake did “as little as possible.” The implication seems to be that no matter how hard you tried to make a difference in Chinatown, you couldn’t, and thus were reduced to doing as little as possible.
Until Jake got involved in the Mulwray case, he also seemed to be doing as little as possible, making a decent living as a marital affairs private investigator. He then gets involved up to his teeth, and despite his best efforts, he can’t make any difference.
It’s telling that the film ends in Chinatown, and Jake’s last line is “as little as possible,” muttered under his breath. And, of course, the last line in the film is: “Forget it, Jake; it’s Chinatown.”