I don't know how the laws work in the UK, but in the US there are hundreds of legal protections and status changes that come with legal marriage (partial list here -- the right to inherit, the right to make medical decisions for your partner, the ability to file income taxes jointly, etc. Being legally married makes all kinds of things much easier.

Maybe the ideal answer would be to drastically alter the legal system, in terms of allowing people to form personal life partnerships, but that's not happening any time soon. Marriage is the best we've got, and that's why I support it.

Now, if your question is "why take a life partner?" that's much more personal and complicated. For me, I suppose it was about stability, and about making a statement to the world that I've chosen to make my life with T. But that's different for everyone.