Which, you know, leads to situations where a complaint is filed, but eg there's fear of retribution. So maybe they'll fill out an affidavit but don't go downtown for an interview. Or they'll do everything by the book, but because they didn't know the badge number of the cop who physically harmed them, the complaint is dropped. This shit happens all the time.
Hell, I've seen complaints get dropped -- despite videos proof attached to the complaint -- that show cops slamming a kid against a wall and threatening to beat the kid up. Because the person getting slammed against a wall didn't know the badge number. There are probably ten different ways to figure out who the thug cop was, but nope.
Point is -- anonymous complaints are important because they give people with real claims of harm the comfort of reduced exposure to retaliation. Not being able to file a complaint because of genuine fear of retaliation is a threatened voice. Anonymity is important.