The main practical reason I've found for caring about online privacy is to make it harder for people to obtain your passwords. For example, don't put your mother's maiden name on Facebook if it's one of the security questions someone can use to get into your bank account, but there are less dumb versions of this.

The main difference between my meatspace and online personas is that I try not to say things online that I anticipate regretting in 20 years, and instead I try to say things that other people will think are cool so they will offer me jobs or something (one reason I use my real name). I'm less filtered in meatspace.