Fame
No, no, don't get your hopes up. Unless you end up as the president of the first big personal robot company, you're not going to be a household name. Even the former head of robotics at Google, Andy Rubin, isn't that famous. Except in the tech world, of course, where he's a total rock star.
Then again, being famous in the tech business isn't a bad option, because it's usually accompanied by a whole Shmoopload of cash. Of course, by that point, your title may no longer be "robotics engineer," but something like "director of robotics," or "CEO."
If you make significant contributions to the field of robotics engineering, there's no doubt you'll be making people's lives easier, and saving people (and corporations) money. That's a good thing, right?