-Under Construction- Bender is a constantly-drinking, hookerbot-chasing, cigar-smoking robot who used to bend girders, and now works at Planet Express, delivering packages with his roommate and human best friend, Fry.
Lots of movies and tv shows had used the idea of a human main character with a robot for a best friend. Sometimes this was presented in a sincere way, sometimes with a wink, but it was one of those sci-fi ideas that just wouldn't go away.
Then the Simpsons happened, and Matt Groening gained this enormous media power where he could go ahead create any new cartoon he chose. Unlike others in his position, he did not just crank out clones like Jango Fett in a military lab. Instead, he took one really good idea, and put most of his attention into it.
Thus, Futurama was born. It was pretty clear that Bender was the most awesome robot friend ever seen, probably starting the first time he ever said "Bite my shiny metal ass!"
Sure, there had been other robot buddy characters before this. We just can't really remember who they are anymore.