Why do I do it? Well, I do it for couple of reasons. First, the stitches look nicer to me, in my humble opinion. Then, if I need to sew the garment together, I actually use a crochet hook instead of needle - I find it easier to crochet edges together. Another reason for slipping the end stitches is that the sides of the item do not stretch on me.
I love that edge especially on my scarfs. So if you ever use any of my patterns, where it tells you about last stitch being slipped, you can make a choice - either knit it your usual way or slip it like I do. Either way is correct.