LE Wilson inside neck reamer?

Nothing because your question isn’t specific. Why are you wanting to ream the inside of your necks? Do they have doughnuts? Are they too thick? Why?

I think the question was more of why does this product exist.

I also saw that product once, and I remember reading what it's purpose was for. Something to do with bushings, brass flow, and bullet seating. However you don't see anybody here talk about it, so I didn't give it another glance.
 
Their purpose is to cut out raised areas inside the neck because these raised areas get pushed to the outside of the neck during bullet seating and then interfere with chambering and bullet release upon firing.
 
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I had a custom size made from Forster for cleaning up inside the necks. I can use bushings and mandrels to determine exactly how much.

I initially got it to clean up some defective brass but now after noticing a donut forming on multi fired brass it will probably be added as a step every other firing or so.