Yeah I really don't get the point my yousemate has one and he insists it cooks certain things better than an oven, but its really just the same and it just seems like a mindless fad people are following that's being pushed to sell useless tat.
My aunt who has been interested in cooking for decades swears they are great. I have yet to do research but I think there's more to it than "smol oven".
We have a small oven, we bought an air-fryer as an additional oven. Kids love how chips (fries) are crispy on the outside and fluffy on the inside with it. I definitely notice a difference compared to my gas oven. Things cook quicker
I think it just depends on what you make. I have one and I find anything that's breaded or fried warms up in that so much better and crispier then the normal oven. Also it usually warms up faster by about half the time sometimes.
I'm guessing it's mostly driven by people renting tiny bedsits where the only cooking facilities they'd normally have room for are a microwave and a kettle. The fad seemed to blow up during the pandemic when everyone's nearby restaurants were shut, so they'd have to live on instant meals and pot noodles. Be a game changer for those folks.
It's probably cheaper to run than a regular fan oven because it's heating less space. Otherwise yeah, it's just a small oven.