For me, it's two reasons.
1) Signing in is a pain and takes time.
2) I can post specific things, and not have people link them together and identify me
For example, I have posted my kid's birth month on the 1/2 birthday thread, and something about his sport at the HS he is going to attend, and something about my neighborhood in the real estate thread . . . and someone could easily put those together and then when I post whining about my husband know exactly which husband I am talking about!