"how many people, when traveling, opt to use the kitchen even once rather than eat out?"

All those with small children, to start with.

Depends on whether I'm traveling or vacationing. For a brief vacation I'll probably eat out every meal. But if I'm traveling, I'll probably want to conserve money plus get tired of big meals. Even on a vacation that goes about a week long I'd probably eat in a kitchen a couple times if I could.

Certainly budget-conscious people too. And some places have great local fish, meat, and produce markets.

Having access to a kitchen may be the main reason that I much prefer AirBnB when traveling. I can't stand being forced to eat every single meal out of the house during an entire trip.

Actually, visiting a local market and cooking at least one meal at "home" sounds like a great break from all the restaurants and cafés.