The brutal reality we live with is that the more posts/info you have online in your own name the more can be misconstrued or improperly used against you, often without knowing it even happened.
Stuff on the web never dies if you want it to. So my posts from a decade or more ago on various forums live on long past my recollection of my username/password and ability to scrub them. Discussions I will have here differ greatly with those I have with fellow rock climbers, backpackers, and such on other forums. Many of those discussions are not what I want popping up if my current or future employer googles me. Even totally innocent discussions can give the impression that I would be rather be outside than stuck in a cubicle (true), which is not the first thought I want a potential manager to have about me.
How many of us would like our "Permanent Record" from grade school to pop up on a google search? How about every product review you have ever done (which indicates a lot of the type of stuff you have bought)? How about any online discussions about your kids, difficulty with kids, health issues with your kids, or health issues having kids? Using online forums to help navigate the world is totally reasonable, but I don't want someone to be able to casually look up EVERYTHING I have ever said on any online forum with ease.
So it is not just anonymity, but using varying online presences to control how much stuff a creepy search will turn up. Tools like Google default to "Full Privacy Invasion Mode" by default, thus I default to "Full Obfuscation Mode" by default.