That's a fair question. I think the biggest benefit would be the recognition of commitment, both by the partners, their families, coworkers, etc., zooming out ad infinitum. It can make a lot of things easier, from a practical perspective--for example, in my line of work there are a lot of couples (shared drive and passion, assortative mating), and employers will often create a second job for the spouse of a person they really want to hire. I think that sort of similar things can apply in other situations. If you, say, want to see your partner in the hospital or have certain other rights it's easier if you're legally bound to said partner.

See the recent arguments about same-sex marriage for a fulsome discussion...

I chose marriage, because I was really f'ing excited about calling my partner my wife. Now, I am really proud to call her that. Note that financially, she is far more well-off than me, but we have similar lifetime earnings potential. YMMV.

Edited: grammar.