I have one so that people can go to my website(s) by typing in an easy to remember address rather than one that's 40 characters long.
www.ianess.com is far easier to remember than : http://ianess.tripod.com/home.html
Pitfalls? It costs money. If it's too similar to another trademark then you may get lawyer's letters.
Even if the trademarked item/service hasn't yet been invented and you named your site first: http://tinyurl.com/3kymx