Well, if you want to use mods which use IDs with 9 digits, you need this.
This is to avoid conflicts more than anything, and secondly so that we don’t have to use Illusion’s slots at all. Lots of mods, particularly from China, use it. It increases the limit of available integer IDs from XXXYYY (6 digits) to XXXYYYYYY (9 digits). MoreSlotID is a patch created by lwlin from ZODGAME (he’s also in HongFire). Please read the frequently asked questions page.