Although I am an atheist, I’ve never ticked the humanist box. I am an atheist because there is no god: humanists seem to want to reify that into a belief.
I don’t have a problem with that. I don’t understand people who do. What matters is not what you believe in, but how you behave towards yourself, other people, and the world in consequence of your beliefs.
Humanist weddings, on the other hand, I completely see the point of. Wanting ceremony and ritual to mark the important moments in your life is a natural human process. Civil registrars provide a ceremony if you want it, of course – within very specific bounds set by the fact that a civil registrar is a civil servant, paid to provide the legal process of marriage, and the small fee paid for a registry office marriage is not intended to allow for any very elaborate ceremony.
If you’re not religious, and many people aren’t (“no religion” was one of the largest categories in Scotland in the 2001 census, so large that the Churches lobbied to have that question removed from the 2011 census) then it’s useful to be able to have a ceremony to mark your marriage that isn’t restricted to a civil registrar’s obligations and that isn’t tied to a God of some description.