For me it's a combination of reasons.
It's a way to thank someone for being a friend and a part of the community. It's a way to let someone know that they really do matter and how much they mean to you. To me personally I think it's really important to tell people how much they mean to you, so a congratulations post is sort of a way of putting someone in the spotlight and showing them that you appreciate them.
If we all just sat and read posts never repling to them, we wouldn't have anything to read, so to congratulate them on X number of posts is like a big thank you for contributing. Thank you for giving your time, sharing your insight, writing things that might have been witty/charming/funny/touching/etc and made the CB a more enjoyable place. Also it can be a form of encouragement, a way to say "Those were some great posts, loved reading them, keep them coming!!!" A congrats post is a great opportunity to say something nice to someone that you might not ordinarily have said and lets them know that they are an important part of the community.
I do miss the congratulations being right on the main CB, though I do understand why they were moved.
Every poster is valuable here, and whether they are just new to the site and have a few posts or have been around for a long time and have lots of posts, it's nice to feel recognized and appreciated.
Overall, a congratulations is like a special thank you to me.
