> Rounded corners also make effective content containers. This is because the rounded corners point inward towards the center of the rectangle. This puts the focus on the contents inside the rectangle. It also makes it easy to see which side belongs to which rectangle when two rectangles are next to each other.

Personally, as a non designer and someone with close to zero natural aesthetic abilities, this single point is the one that resonates. There are some forum tools that used to choose to separate content threads by rectangles. When scrolling through a page quickly it was easy to miss where one thread started and the other finished. After the switch to rounded corners the distinction when moving fast became a lot easier to see.