All of what Brad said are true and important. I'd just add that it's a "do unto others" sorta thing. HTTPS reduces your user's online footprint. People who may be snooping on your users will still see the site they're going to, but not the content. Not what they submit to the sites, etc. Even the most innocuous things can be used against people, so the smallest possible online footprint is a good thing.
And I'd add that if you add Strict Transport (HSTS), then it's much harder for anyone to forge your site.
So I'd reduce it down to 1) protecting your users, 2) making life difficult for the bad guys and 3) making your site a difficult target.