When you get a good answer, it is extremely helpful to everyone if you mark it as a correct answer. A hundred people may look at a hot topic, and when there is an answer, they can jump straight to it. So for everyone's benefit, when you get a good answer, mark it! It's part of being a good community member.
As well, but much less importantly, one of the things we're going to measure is how quickly the community answers questions. But we're far more interested in people being able to find the right answer. So from time to time, if it looks like people forget to mark answers, we may go through and do it for you. But we aren't your mom. Help your colleagues, and mark the good answers. In fact, you know better than us what's right.