Picture a 3x3 grid of pixels like this:

O O O
O O O
O O O

How would you color in those pixels to draw a complex shape like the letter "P"? You really can't. No matter which way you choose to draw it, you'll either be missing the bottom of the stem, the hole in the middle, or something else. Similarly if we were to try and draw a diagonal line "\" it would have to be stair-stepped.

Computers try to get around this by doing something called "anti-aliasing," which adds translucent pixels around the edges, but it's more of a bandaid for the problem since it relies on the computer trying to guess about missing information, and often results it trade offs like smoother edges but tiny details vanish since they are too small to be rendered in the first place. By rendering at a higher resolution and then scaling down, we can render all the extra detail that needs to be there, and then scale down to get an "accurate," image.