For online forums website
A forum is basically an online community of people will similar interest connecting them as one. Buying a .COMMUNITY domain for your online forum is just another way if making it clear that your forum is aimed at promoting a particular community of choice. The domain name would also serve to instill the spirit of community into the minds of the forum users.
For social media websites
Just like forums social network also serves as a connector of people online. It is an online community with a much buggers scope and coverage than that if forums in most cases. Social networking websites can also enjoy from the use if the .COMMUNITY domain for its websites.
For community related programs
Your community might be doing a festival, party, or even a rally. Buying the .COMMUNITY extension for the events website could foster the spirit of togetherness that such an event needs to thrive effectively. Also adding the world community to you domain also paint the picture of multitudes and this could just be force needed improve the overall outlook of the events.