There have been threads in the past about gunsmiths who will turn the gp100 into a 10mm revolver. And I am sure there are other calibers that could be used, don't know what is reasonable and what is not though.
So having the spare parts available for the modification might be nice.
I also guess if someone wanted to really modify a cylinder for moon clips and chamfering for quick reloads it might be nice to have a spare around.
My first thought was 357 maximum loads on a sane level that the gp100 can handle but I have never dealt with the 357 maximum so I don't even know if they would fit in a gp100 cylinder.
I can see reasons to have the kit, right now I am trying to make sure I have enough cash to cover my bills for this winter since I don't have a job so the kit is a luxury to consider but I don't think I have a need for it.
Overall it is something to consider if you are seriously interested in being able to work on your own gun since it gives you some parts to experiment with.
Please note this post is just random thoughts and is not intended to say any of the above is safe or sane in any way.