The short and simple answer to why I feed F/T to my snakes is this: it's safer for the snakes.
There are several reasons, but that's number one.
FACT: Dead mice and rats don't fight back.
Aside from breeding my own mice and rats buying F/T in bulk is far and away the most inexpensive way to go as well.
It's also convenient. Buy prey items, put prey items in freezer, take out what you need. Order more when getting low. Simple and delivered to my door.
I personally love pretty much all animals, mice and rats included. So watching and hearing them die isn't something I want to do. I eat meat, I know where it came from. But at least I didn't have to watch it die. When I buy groceries, I just see food. When I buy F/T prey I just see food. In either scenario, it's food to me at that point and not a living animal.
Anyway, those are the main reasons I feed F/T to my snakes, with safety to the snakes being the most important factor.