When you're working with rounded objects whose edges have been smoothed, it's sometimes hard to make things look good. You can see what I'm talking about in the images that follow; notice the (what I consider to be) unsatisfying outline of each of the rounded objects below?
Without Profiles turned on, rounded objects don't stand out:
Turning on Profiles in Styles dialog box produces a completely different result. At a Profiles setting of 2 pixels, perimeter edges become clearly visible. Profiles that are 2 pixels thick often look too bold and cartoony:
Dialing down Profiles to 1 pixel solves the problem (see below).
Using a Profile thickness of 1 pixel makes rounded objects pop out from the background.
It's worth mentioning that telling SketchUp to draw Profiles can slow things down considerably if your model's pushing the limits of your polygon budget. It's better to switch Profiles on when you need them.