I've already used up all 8 of my characters slots, though I only tend to play as 1 or 2 of them at a time. I created them mostly because there weren't a lot of buildings and my Island was looking bare bones, especially my city area.
I figured, along the way, that I could also use them as extra storage space, once I started collecting more clothing and items because I'm a filthy hoarder and I started running out of space.
At the time of writing this:
•Character 1 stores favorite clothing and a few important crafting materials I usually always use up
Character 2 collects villager photos and villager posters
Character 3 collects crafting materials 5 stacks of each in case I ever need it
Character 4 is collecting all wallpapers, flooring and fish/bug models
Character 5 is collecting furniture I craft with my MC
Character 6 is collecting bugs and fish to give to Flick/CJ for models
Character 7 is collecting any extras
Character (I haven't decided yet, but probably more non orderable items)