Think of them as something that allows the overall screen area to increase, as bevels shrink.
And then when the corners of the screen are so close to the corner of the laptop, and the corner of the laptop is rounded, it looks weird if the corner of the screen isn't rounded. Like a square peg in a round hole.
Fortunately, it's all context-dependent. So when you watch a video that is inherently rectangular on a Mac, that takes precedence. It's only shown below the notch, and the rounded corners on the bottom disappear.
So it's kind of the best of all worlds. Bigger screen with round corners for actual work (the notch is not particularly objectionable in the menu bar), slightly smaller screen with rectangular corners for video (and games too I assume?).