Both scenes mark a major step in Fleck's transition from a strange-but-friendly clown to a clown who does Joker crimes. But mostly they're there to show you that this dude is CRAZY. I mean, he's headbutting things! What kind of sane man (who doesn't fight in UFC) does that? And weirdly enough, he's not even our only clown example.
In the second It, Pennywise flexes his crazy by methodically smashing through a plate-glass window with his bulbous head. Of course, he then goes on to eat a child -- an activity primarily reserved for the mentally unstable -- but I've seen clowns eat children dozens of times before. It's the headbutting that sticks out as a modern move.
But it's not just clowns who butt heads. As everybody in Bird Box loses their minds upon glimpsing aliens flashing their taint or whatever, what does the very first crazy person do? Headbutt a damn window. And because my own brain is broken, every single one of these scenes reminds me of feral vampire zombie things in I Am Legend headbutting Will Smith's various windows.