If you want to draw squares, you're probably looking for a vector drawing program, like Inkscape.
There are hundreds of good reasons why someone might want to overlay a vector shape on a bitmap image. The desire to draw shapes on bitmap isn’t something weird that I’ve just invented for HN. It’s been a staple feature such graphics packages since the inception of bitmap graphics editing. And it’s been a staple feature of Gimp since I first switched to Linux in the 90s.
But that’s all moot because I was just making an arbitrary example.
And as an aside, I do use vector drawing software too. So I’m fully aware of their existence.