Not all number 8s do it, but some do. It is simply to provide extra cover on the back, and also to offer more running options. Also number 8s in super rugby are quite quick, so again gives more running options! Hope this helps!
Number 8's in the S15 are very versatile and position very well. It's a new way of playing for the number 8 as there is more emphasis loose forwards actually playing a little more 'loose'. They can run from the back, and provide extra cover with wings and full backs. You would be surprised to come watch a New Zealand High School game and see many number 8's doing the same thing.
