A lot of the good (from the police perspective) is that people don't act like jerks when they're clearly being filmed. Amazingly you're less likely to be a dick to cops when the camera is on you. In-car cameras turn on and off automatically when they have the lights and sirens on. Pull a guy over, and video gets shot - period. Wearables don't have that yet, but we'll get there.
That was in fact the main point of the ACLU's objection (RTFA for details), citing that the law did not require the devices be turned on, did not specify that citizens could access the recordings, and did not specify data retention policies.
In TFA the ACLU cited several actual court cases where cameras were running, and then either mysteriously malfunctioned or were intentionally turned off by police or where the footage was "lost", and the judge ruled in favor of the cops because of a lack of evidence and they believed the officer story.
The bill only said police had to carry the cameras. I'm certain the good cops would have left them recording and done all they should. Unfortunately the law allows bad cops to keep turning off the cameras during the beatings, allows bad cops to retroactively edit and delete the video, and allows departments to keep the video secret unless government prosecutors review the recording and decide it contains potentially exculpatory evidence. The ACLU wants mandatory recording, mandatory citizen access for their interaction with police, and mandatory retention policies.
That avoids the "oops, I accidentally turned it off while you were beaten", and the "you need a full lawsuit to get access to the clips, assuming we still have them [wink]", and officers deleting the clips of the beatings so they can claim malfunction later. These are standard specification requirements in data systems, so the fact that the three were missing from the bill was quite suspicious.
(Basically bad cops and prosecutors could continue to use the law against people. If these prosecutors review the video and believe it doesn't meet the exculpatory rules the current law says they don't need to hand it over to defense, although fortunately more judges are starting to rule this violates due process, telling cops and prosecutors to hand over EVRYTHING instead of just LEO-submitted evidence and LEO-censored exculpatory material.)