I support avoiding redundancy in general, but there are advantages to those that make them worth it if you get enough of a quality of life improvement for them.

I use my air fryer basically every day and really appreciate that it can finish cooking some things before my oven would even finish preheating. Cleaning is also much easier, I imagine it uses considerably less energy, and it tends to cook stuff more evenly in my experience. And it isn't some fancy product, just the cheapest one I could find when I decided to try one out years ago. I've used my oven for a couple of things that wouldn't fit in the air fryer over the years, but otherwise it's basically been reduced to a stovetop with a clock.

I also use it for things I would otherwise microwave because it cooks it much better even if i have to deal with a short preheat. So nowadays my microwave is just left unplugged until I need a quick injection of heat, like for freezer-burnt ice cream or melting some butter or cheese.

I don't think I'd get enough use out of a slow cooker to commit to the space, but I imagine it's a similar story for people that would use it often. Same deal for like a rice cooker or ice cream machine. Also, my family was able to get an old toaster oven for cheap after our real oven broke, so these appliances can offer a cheaper alternative too.