There are very few cases where there's a good reason for a printer of any kind to be on WiFi and even less for a receipt printer. If it's being used in a portable application with a laptop or mobile device that's what USB or Bluetooth are for. If it's sitting on a checkout counter and needs to be shared between multiple PCs that's what ethernet is for.

I'm not saying that there are absolutely no situations where WiFi is actually beneficial in a printer, but most of the time that a printer is connected to WiFi it's just making the printer less reliable than it could be if it was connected another way for no reason other than the user not liking wires.

A universal truth of networking: If it can be practically wired it should be wired. Wireless is for things that move and things that need to be put in weird spots it doesn't make sense to ever wire.

A good reason is that it's easy. Turn it on, connect to wifi, done.

That's not always an option, and doesn't answer the question.

Some people rent and can't simply run Ethernet everywhere.

Recently I learned there are Wi-Fi printers without Ethernet port!