If you have a help forum, for example, and you want the user to populate certain fields so that you have all the information they need.
If you have a gaming community, and you want people to enter the game they are referring to.
if you have a community around nature (like bird-watching) and people are uploading images and you want to make sure they enter things like the location of the image and/or the subject of the image.
I'm sure I can think of other examples if I put my mind to it. The idea is to eliminate things like "if you are posting in this forum, please make sure you provide the specs of your PC". That goes away, because the user simply can't post anymore without providing that information.