That "grandkid" that got rescued was the child Saitama rescued in episode 1. His grandfather (if you could see that file) also has a "butt-chin". In effect, the Hero Association only exists because of the actions of Saitama when he was still a businessman.
EDIT: I guess I got beat on this point
Because its funny
EDIT: I guess I got beat on this point
Why does Saitama have a hoodie that says "boobs" in Japanese?
Because its funny