I threw one in the trunk of my car (just in case - I ordered a 4-pack and I had a spare one), and every single time I drive somewhere it chirps loudly when I'm exiting my driveway, making its presence immediately obvious without any delays, and despite my phone being with me in the car.
For small items where the airtag is merely present, it can be useful from the perspective of being alerted when an item is left behind or taken away from your surroundings. Much theft crime is opportunistic, such as forgetting a wallet behind in a restaurant, or dropping it in a street. Airtags are useful in that regard as the "left behind" alerts are reasonably timely, and tracking an item down works well. This works because you realise it's gone and find it before someone else does.
While there are still plenty of examples of people using airtags for recovery of stolen goods, that's not the main product intention as it is very easy to discover and disable a loose airtag. Some larger items however (e.g. certain bikes) have airtags built in which aren't easily disabled, there are also mounting cages which serve a similar purpose - in these circumstances it is a legitimate antitheft device because the ability to easily disable it is removed.
I use airtags and my most common use of it is to ask siri to "jingle my keys", which alerts me to whichever coat or pair of jeans I've left them in (my second most common use is receiving an alert that I've left my umbrella behind somewhere).
I also remember one occasion of leaving my keys in a taxi, realising immediately, and chasing it down the road - the taxi never saw me, and I never saw my keys again, with an airtag that would have played out differently. These kinds of situations are far more common than dealing with thievery.