drop bear
Sr. Grandmaster
I'd consider this training method (not the techniques, but the training) similar to the focus mitts. There is an assumption of response/situation in that. The trainer provides a set of targets and responses that's repetitive (in repetitive drills - obviously, mitts don't have to be repetitive). The issue - and I think it's what you're getting at - is if there's no sparring, they'll never know if any of these transitions are actually likely. Whereas, because they spar so often, boxers are pretty aware the combinations they train are responding to things that actually happen from time to time.
So, let me be clear, I think this fills the same function as one part of how a boxer learns a combo. There are some reasonable arguments to be made about the advantages of mitts (speed, using some power, etc.) and the advantages of using a person (can see what your block would actually contact, etc.) and whether the trade-off is worth it.
Boxers don't fake the outcome of a punch successfully. So you don't go from win to win which is nice but not very useful. As learning to towel up a guy who is getting rocked isn't high percentage.
So what they do is fake the defence. You punch, they slip and counter. Then you counter off that. Because, well that is handier to know.