I guess some people like to share who they are with the world. Depends on the person, but sometimes it's because they've faced adversity and are proud of it, sometimes it's for attention, sometimes it's to avoid that awkward conversation when you want to bring home your same-sex partner to Thanksgiving and you don't want your grandma to have a stroke at the table.
It's a matter of personal choice; you don't have to come out if you don't want to. With the increase in visible prejudice and unrest within the community, a lot of people are 'coming out' to make a stand.