IR blaster. I could control just about anything in my house with my old Galaxy S6. Made it so convenient to have a universal remote built into the phone. Especially when you end up in a hotel or at a friend's house and can't find a remote.
Oh, my HTC One M8 had that. Really miss it for the IR Blaster and the air pressure sensor. You always had a really good weather station in your pocket.
I used to love having one of these while I was in college. I would use it whenever I went into the cafeteria for lunch and they had Cartoon Network on on the TV and Teen Titans NO was on. Yes, I know what I did. It is the only appropriate title for that steaming pile of radioactive waste.
It was also fun to mess with other public TVs when they thought that nobody would have the ability to change the channel.