I don't think public figures come out for attention, as the media makes it out, but so that some LGBT kid in the Bible Belt who may be contemplating ending his or her life has hope! They too can be an actor or actress! They too can be an Olympian! They too can be anything they want to be.
To today's LGBTQ youth who look up to out celebrities like Tom, the idea that they are "allowed" to entertain the same romantic fantasies as their straight friends is a big deal. I know that having someone like Tom on my radar would have made a big difference to me growing up.
Visibility is why coming out is still a vital part of the fight for equality. Having a high-profile LGBT person come into people's living rooms daily and doing their job well can help remove some of the "otherness" of gay people and increase cultural comfort levels.
Luke Evans has the talent, not to mention the biceps. Having appeared in Clash of the Titans and the recent Three Musketeers, and with a part in the upcoming Hobbit movies, his Hollywood resume is filling out nicely. So why now hide his sexuality?
Appearances do matter -- public appearances, the kind that a confused and lonely kid in Russia or Kansas or your neighborhood might see. "It gets better" is a nice but passive sentiment; come out, go out and help make it better.