Cupholders, and they come in multi-set setups with each person having their own seat. So if you're building an actual home theater so you can watch movies with friends like you're actually in a theater, so they give a better movie watching experience. If your home theater consists of just a flat panel TV and a surround sound system in your living room, then you don't need them.
Though a home theater forum would probably be a better place to ask that, since you haven't really given us any information as to why this is important to you.