It depends on what kind of trip you're booking. If something complex, with multiple separate airlines/tickets and various hotel bookings, there is always a chance that one of these booking processes may error out. Prices may jump up while booking. When you have expedia's payment page say "Sorry, this itinerary just went up $1000" its awfully nice to have the option to reconsider your trip and be allowed to cancel the already booked flights.

Same for when you find a great flight deal with very low availability. Book the flight first, then look for hotel options. Sometimes hotels are so expensive that it makes more sense to then choose to fly another date, even if flights are a little more expensive then. So at that point it makes sense to cancel that flight booking within 24h.

Ideally, you'd have the option to create one booking reference (e.g. Amadeus), and add your flights, hotels etc to that booking - reserving them in the process. And pay the full balance, and fully commit to the full itinerary, at the end - at once. Kind of like a database transaction, where you commit all changes at the end (or rollback all changes if there is an issue at any step). But unfortunately our travel industry is not set up for that yet, at least not for people not working with/for a TA.