There was a time when rounded corners were a challenge. The box model ruled and everything the web was boxy. Rounded corners and circular images stood out because of the extra effort needed to create them. PNG images with transparent backgrounds weren’t quite there yet. Some browsers, and yes I’m talking to you IE, took longer to get them right.
Curves are much easier to create today but I find that they still stand out in a design. If you are designing something, try thinking outside of the box but don’t go crazy!