I know it's overused and for example in MacOS introduces way too much white space, but people like rounded shapes. They make people more comfortable and make them think the content is simpler than when they get hard angles. It's been tested in so many research papers and the basically all agree - people like bouba more than kiki, whatever the context is.
For me the great benefit of rounded shapes is that they make it visually apparent what's foreground and what's background. If you just divide an area up into rectangular subareas, it's hard to know which of these rectangles are supposed to represent figure and which are just empty background. The T-joint between a vertical line and a horizontal line gives no clue as to whether the vertical line is supposed to connect to the left half of the horizontal line, the right half, both, or neither. A little curvature makes it clear which direction the edge of a depicted object is continuing.
On a similar note, fuck the "flat" designs which make buttons indistinguishable.
I've even seen UIs which do use bevels on buttons; but only when hovered-over! I don't want to scan my pointer across the screen hoping to find something interactive, like I'm struggling on Monkey Island!
