Purpose of allen head "indexing" screw in 5c collet block

[JMI]

Purchased a set of Enco collets and collet blocks a few weeks ago and it has made work holding in many cases a piece of cake. How did I ever get along without it.
Was wondering the purpose of the small screw towards the back of the collet block that fits a groove that runs the length of the collet?

Thanks

Jim

 

[Powder keg]

It keeps the collet from spinning in the holder)