I like the feedback from a broad range of perspectives. I'm a pretty straight-arrow, traditional, midwestern person surrounded by a lot of people fairly similar to me. I can often predict what my Mom, or my IRL friends, will say about things in my life. But on MDC, I get feedback from all over the place - geographically, sociologically, philosophically. Sometimes, another member's perspective will be something I never would have expected - or come up with on my own - yet when I think about it, it makes sense.
Also, reading other people's issues and thinking about how to be helpful in response is an excellent mental exercise, for examining your own values, priorities and beliefs.
It's healthy, too, to have an outlet for discussing things besides your partner. My husband and I are great friends and talk about everything, but, for example, he had a very difficult custody battle over his son from a previous marriage. For a long time, that was (unavoidably) a main topic of conversation between us. Once he "won" custody, I continued to think about those issues a lot - as well as all the new issues that accompany helping to raise another woman's child. But it didn't feel healthy for our marriage, to let his ex-wife continue being such a big topic, after the biggest problem with her was solved. So DH and I still discuss important issues about his ex, but I explore a lot of things here, too, so DH and I can mainly focus on "regular" family life.
And let's be honest, sometimes when you have children it's easier to conduct online friendships. I mean, you need the regular kind, too! But it's very conducive to motherhood, to be able to "listen" and respond to friends' thoughts when you have the time; and to be able to stop, mid-message, and return when it suits you, without having to make any explanations or apologies or feel torn between your screaming toddler and a friend in need.