As the title says, what does it do? I obviously do not mean complete removal of culpability where a mortal sin does not qualify as a mortal sin, but rather culpability not taken away entirely but still mitigated.
As an example, a person daydreams about grave matter (violence, lust etc) mindlessly and only snaps back once they realize properly what they're doing. Anyone who has daydreams can tell you clearly that in such a state the ability to consent is diminished, but you still have some awareness since you aren't in a state such as one who sleeps. So, what does the person do now? Mitigated culpability doesn't mean the sin is venial, so should one assume they're in a state of mortal sin or not? Should they take communion? (I say all of this for confession taking place after mass, as is the norm).
It's a gray area amongst moralists with no clear answer, so why even introduce it to laity? What does it bring one to know that you aren't entirely culpable for the act if it's still mortal? What "benefit" does one get from this? I feel as though this worsens scrupulosity, unless there is something that a person can use to determine if he is exempt from it being mortal or not. Even so, if you are exempt from it being a mortal sin there still is culpability you need to carry.
Quite frankly, I just find it unnecessary. Sin isn't black and white, culpability can be mitigated and I'm not saying it's not possible, it very much is, but rather that it just doesn't do much. Either one can tell when he is exempt from mortal or one can tell he isn't exempt from it being mortal. Adding this gray bridge where if you stand there it could be venial or it could not is simply horrible for lay people living in a world where regular confession sometimes don't even take place. Of course, unless I am missing something about migitated culpability, which is why I'm posting this.