If you don't normally use Curves in your adjustments, but instead prefer to use the levels tool, you may be wondering what advantage it is to use Curves. There are several reasons to use Curves, but Levels can be very useful too. Aperture's levels tool is actually pretty powerful, and you can do a lot with it. In many ways they both perform the same task, but approach it in different ways. Personally though, I prefer to use the Curves tool.
However, Levels does have a few disadvantages. For a start, you can't access the extended range data with Levels. It only works with the visible range, so you can't use it to recover highlights. You also only have basic control over how the curve is interpolated between points. Another important factor is that, with Curves, you get a very clear visual representation of what you are doing to your image.
The one big advantage of the levels tool is that it is fairly low-down in the adjustment tree. This means that...