Here's my guess: When you bind off a stitch, you create a little bump on the end of the row where the bind off is. A decrease in the near end of the row (usually at least 1 or 2 stitches in from the end) reduces the number of stitches in the row, but does not create the bump. Makes for a more graceful transition.
Also, your decreases can be decorative, and add a nice touch to your garment.
Just a guess, but there you go.