I imagine it's because your anatomy is actually more suited to dropping a log while popping a squat. Because of this many people will keep a stepping stool in front of their toilet to put their feet on while sitting on the toilet. It can make doing your business easier, especially if you're constipated

You don't have a poop step!?

Sometimes you squatty for the potty

Some people have them so it's more comfortable because the toilet is too high.

Okay it's a pooping step, I'm aware of those. But why is it blue?! I guess the source image was probably a thing about pooping steps. So it was originally blue to draw attention to it. Well it worked!