Not the OP but I own some ab.xy type of domain names and use one of them as my personal gTLD.

It's like having infinite "free" domains (even with the small fee for the base domain.)

But the most important part is the fun of just having an entire namespace at your disposal to create whatever "domain name" in seconds.

Do you mean you can easily spin up foo.ab.xy, bar.ab.xy and baz.ab.xy?

Yep. I recently built a noise generator app and hosted it at noise.ab.xy.

Nothing technically groundbreaking (actually Cloudflare does it automatically for me), but it's a nice quality of life trick.

This was written assuming you already have a domain. If not, then yes that would be an additional cost.