Many hotel booking sites do charge at the time of booking, particularly when the booking is heavily discounted / non-refundable.

Depends. Many sites offer non-refundable reservations for a cheaper rate. You pay when you book.

booking.com always charges me before I fly. Same with agoda and another site I’ve used which name eludes me.

Never had the option to pay at the hotel upon arrival.

the absolute best fares i got were always paying in full before the actual trip. i still do this today with covid.

A lot of trips and stays are prepaid in exchange for a lower price.

Lots of hospitality businesses charge before the stay, like travel agencies. A company may need to reserve flights, book rooms and possibly set up tours and events, and all that stuff costs money upfront. If you don't have a lot of buffer (say, because Stripe is reserving half your funds) then you'll need to cover the costs in some other way.

If you only book a hotel then you'll probably get the option to pay upfront or settle afterwards, but if you buy an entire package deal from a third party comaonty then you usually get asked for some or all money upfront. These packages are often more expensive than going through the effort of finding a good deal yourself, or they're cheaper and rely on a lot of deals not falling through.

I wouldn't trust my customers to show up without at least a reservation on their account, especially now with new covid waves and potential anti covid measures threatening holiday fun.

Booking.com, expedia etc. have options to pay in advance. They'll refund you later if you cancel. Some listings seem to have this as the only option? I didn't look recently.