Gem slots for loadouts are extremely limited. With so few options, it can be difficult to tailor different items towards your build.
"More Reasonable Slots" seeks to overcome that restriction by rebalancing how slots are arranged across every item type, creating an overall increase in the number of gems to use.
Not interested in balance and want to go totally hog wild with no restrictions at all? There's a much more ridiculous version of this mod titled — and no, I'm not making this up — "Slots For Each Item And Each Item Has Max Slots Or How I Learned to Stop Worrying And Love The Slots."
That version is most definitely a cheat rather than any sort of rebalance, as it gives every single item the most possible slots. And yeah, its a lot of fun to play that way.