When you K2tog, the stitches slant from the lower left to the upper right. When you SSK, the stitches slant from the lower right to the upper left. So if, for example, you are doing decreases on both edges of a sweater or a mitten and you wanted it to look like this:
// \\ instead of // //
you'd use the different techniques on the different sides. They both decrease, but it looks nicer if you make the stitches lay the direction that they should.
Once again, I'm sure that I'm clear as mud, but trust me that it does make a difference.