Ida Sessions is a "working girl"—i.e., a prostitute. Noah Cross and his collaborators hire her to pretend to be Evelyn Mulwray and to ask Jake Gittes to investigate Evelyn's husband, Hollis, for adultery. When Ida later discovers that she's been used to frame Hollis for adultery and make his murder look like a suicide, she calls Jake and gives him a crucial clue.
Somehow, she knows that Noah is buying land in the Northwest Valley (where he plans to store the water) and registering it under the names of retirement home residents, some of whom are already dead. She directs Jake to the newspaper's obituary page, which has the obit for one of these retirement home residents.
This is of major importance, since Jake is then able to match up the retirement home residents' names with the names of the landowners in the Northwest Valley.
Unfortunately, the next time we see Ida, she's dead. Cross's henchmen have apparently killed her, and the cops have already beat Jake to the scene. This demonstrates the movie's dark message: bad guys thrive, while people who try to revolt against evil are crushed like a saltine cracker until the heel of a Doc Marten.