Apple's Technical Q&A on addressing flickering (QA1650) includes the following paragraph. (Emphasis mine.)

You must provide a color to every pixel on the screen. At the beginning of your drawing code, it is a good idea to use glClear() to initialize the color buffer. A full-screen clear of each of your color, depth, and stencil buffers (if you're using them) at the start of a frame can also generally improve your application's performance.

On other platforms, I've always found it to be an optimization to not clear the color buffer if you're going to draw to every pixel. (Why waste time filling the color buffer if you're just going to overwrite that clear color?)

How can a call to glClear() improve performance?