As other have said, they're usually not the same shoes I wear outside.
But generally, for the same reason I wear them outside: to protect my feet. Especially if I'm going in and out a lot, like when doing yardwork or grilling.
Every time this question comes up, I can't tell if it's that homes outside the US are much cleaner or streets are much dirtier.
I wear slippers inside mostly to protect agains cold floor, coffee tables, and most important of all, Lego bricks on the loose.
Also another reason to wear shoes inside is when you are constantly going inside and outside. Which means then your floor is dirty.... which means you want to protect your feet from the dirt. That's a vicious cycle but can be one of the reasons.
This is what Crocs were invented for. Just slip your feet into “outdoor slippers” on your way out, and kick them off on your way back in. You don’t even have to search for matching socks, and it still works in the rain
Loose nails in the floorboards, kids' toys, yes table legs, things dropping on them from height, cold floors, cold air, pouncing cats, slipping in puddles, hot oil spatter in the kitchen... life in a 140-year-old house with three kids and five cats, basically.
In that case I perfectly understand you. Society really doesn't take coffee table attacks as seriously as they should, really vicious things if left on their own.