I think its suppose to protect the valve body if you bottom out the stock. Like if the bump stop compresses all the way and hits the strut mount.
Edit: Some also say that its also to protect from dirt and other crap from getting into the valve body.... Also have seen that its so that the valve body doesn't create a vacuum on the rubber bump stop which would not allow the strut to extend correctly.
The white ring on your KONIs is called the "bump plate" and is a surface for your bump rubber to impact into that will keep it from pushing an air pocket or road dirt into the damper seal. Although a simple little piece, it is helpful to provide a little added protection for the top seal.
Sounds like a good idea to have it, especially for warranty purposes.
Put them on.
You DO NOT have to pull the shock to do it....
That is why they are split, if the ones you have are not split, just cut them with a pair of scissors and put them on that way.