Firstly, many of them have a backrest for pillion (the main reason I chopped in my R6 for this is to take the long haired General out).
Secondly, I like the thought of locking the helmet(s) away and not have to carry them about on day trips and the like.
I also installed the OEM rack and bought the 39L Yammie Topbox. The rack fits well, was easy to install and eliminates the need for a tail tidy. My wife likes having the topbox behind when pillioning and was desperate for the grab rails which was one of the motivating factors in getting the Yamaha one.
It is simple to find where to drill the holes as the plastic panel are marked for them, you do need a pretty big drill though, 25 or 27mm I think, something like that.
Yes, the hole should be 27mm which is an odd size. No-one makes a holesaw that size. I drilled it out to 25mm using one of those conical step drills and filed out the rest until it fitted nicely. As Trevor said, the holes are marked with a cross where you need to drill.