It is for items that could get lost / misplaced. In a general sense things likes keys, luggage, backpacks, wallets etc. do get left behind (or in the case of luggage "lost")
I know people who have misplaced keys inside their own house. They were left in a pants or jacket pocket, in the clothes hamper, etc.
Obviously there are people who are very perceptive / organized so they'd be more aware. There are just as many people (or more) who leave or misplace things. It seems so unlikely that a person wouldn't notice, but it happens.
A look at any "Lost & Found" section at a mall, theme park, hotel, movie theater, etc. shows there are a TON of lost and forgotten items.
I lost my car keys at at AMC once. It was during a cooler season, so I wore a jacket & at some point during the movie my keys fell out of my jacket pocket (Reclining seats + loud movie explosions apparently made it easy for them to fall out of my pocket with me knowing it). By the time I got to my car I realized I didn't have them, but we had made a few stops at other shops, so in a panic, we had to retrace our steps.
For me, it's more of a (cheap) additional level of protection. I always put AirTags in my luggage when I travel, so if my luggage gets left behind / lost. It makes it easier to show the airline rep where it is (And it definitely works in those regards)
My goal is to NOT have to use AirTags (because that means I misplaced something) but I have them in the event I need them.