You can meet other designers, ask questions, talk about the latest design news, ask for critiques of your work and much more. Being active in a design community is also another great way to promote your own work and get your name out there. If you are an active member and you contribute quality posts, many forums will let you add a signature after your posts linking to your portfolio.
Don’t spam forums with links to your site or most forum moderators will ban you right away. If you are going to join a forums, join with the intention of becoming part of a new community.