I don't really gain anything from seeing it in the theater versus watching it on the TV or on my iPad.
That's mostly true, but not entirely. Most films don't really benefit on having the larger screen, or 3D, or rumbling seats, or spraying your face with water or whatever they do. But that doesn't mean there's no benefit to the theatre setting.
Further up in the discussion whoever57 said "You just don't have a good enough A/V system at your house." Most people don't. I know I don't. Either the viewing angle's off, or there's too much light, or the sound mix isn't great, or whatever. On top of all that, sitting and watching a movie in your living room leaves you prone to distraction. Part of that comes down to discipline, and taking the time to set things up as right as possible within your means, but sometimes that's asking a lot of people, and isn't always manageable.
A theatre isolates and insulates you. It's just you, the audience, and the film. You can (and yes this is a terrible, overused term) immerse yourself in what you're watching. While audiences can be good or bad, watching with the right* audience elevates a film. Ultimately you focus on the movie and, assuming it's not crap, can enjoy it more.
Nothing that can't be done outside of a real (and expensive) home theatre set-up, it's worth pointing out, but as I said above that's not realistic for most people.
*what makes a "right" audience is entirely subjective, and based on the movie. I watched Grindhouse and The Raid 2 with audiences that hooted and cheered throughout, and it was perfect. I was the only person in the theatre for Soderberg's Solaris and that was also perfect.