This is actually a pretty complicated question...
Some drills/kata/exercises are done against specific, somewhat unrealistic or unlikely attacks in order to practice a particular technique or principle. My favorite example of this sort of thing are techniques done against lapel grabs... Really, who grabs a lapel with any serious intent anymore? But what is that "lapel grab" actually defending against? Would the same principle work done against a punch that you just faded back enough from? So, what is that "unrealistic lunging punch" actually doing? Could the principle be applied in a different situation? What's underneath the obvious movement? And, sometimes, the "unrealistic attack" makes perfect sense if you look into the origins or where the art came from -- like if it's based on someone wearing feudal era Japanese armor versus one wearing a boxer's shorts... Each is going to move differenly, right?
But there's more to it... because a lot of the time, the attack IS unrealistic, even as what it's "supposed to be." How often do we see someone practice by "feed the punch and stand still so that many bad things can happen"? Often with a punch or kick that isn't even close to actually hitting anything? Not good... Not good at all. Each training partner has a job to do, and the one giving the attack is supposed to be giving something workably real to deal with. At first, that may be "stick out out there like a statue" move, but it should still be in range, and have enough authority to enforce the desired response. Then, as the practice advances, the attack should become more fluid, and less cooperative. It's vital that you be fed a realistic attack if you want to be able to develop a realistic technique.