IME, they are used for the kind of meeting that isn't really a meeting at all. Nothing is discussed, and some idea is "presented" to a Powerful Person by the Presenting Person, plus some other people who are there to spectate for no obvious reason. At the end of it, Powerful Person says they will think about it, and Presenting Person will send out a summary of the meeting in email.
Why not just send the email to begin with, with a full, coherent summary of the proposal, have Powerful Person email their approval or rejection, and skip the several person-days worth of meetings?
First, Presenting Person, like nearly all people, is borderline illiterate, hasn't written a report or essay since college, and probably simply couldn't do so coherently in any reasonable length of time. For them, PowerPoint is a crutch to avoid having to actually commit their proposal to writing. Instead, they put up some bullet points that are vaguely related and just talk about it.
Second, there is Powerful Person. They are again, like most people, borderline illiterate, and haven't read anything longer that a listicle since B-school, so they'd rather sit back and stroke their beard for 20 minutes than bother with possibly needing to look up a word they don't understand. More importantly, the presentation or "meeting" is an opportunity to display primate dominance. Essentially they are "holding court" to demonstrate to everyone that they are the ones in charge. The idea that Presenter Person may have stayed up late getting every slide pixel-perfect in preparation for this meeting only enhances this function.
Finally, there are the spectators. Powerful Person needs them there for the primate dominance function already discussed. The more people there are to witness Presenting Person prostrating themselves before Powerful Person, the greater utility the ceremony has in this regard. Luckily, all of these people are easily convinced to show up for the meeting, because it's a way to schedule a surreptitious break into a workday that has probably had all actual breaks like lunch breaks, coffee breaks, cigarette breaks, slowly eliminated over the course of years and decades.
In short, nobody can read, nobody can write, and showing up for meetings is easier than real work. Enter PowerPoint: the software package tool-using social primates reach for most when nominally attempting to make business decisions.