I mean, I've beaten Starward Rogue at the highest difficulty... with the redshift, which by default dies in one to two hits, but gives you all the time in the world to figure out what you need to do. It's not really a matter of understanding, so much as processing what you understand fast enough to react at the pace the game is asking for with the precision it's asking for.
Some folks by and large just can't do it beyond a point that's often enough well below what the kinds of games in question consider normal or easy. Slowing it down can make it so that sort of fellow can go from getting stomped on the lowest difficulties (basically stonewalled out of the genre) to successfully navigating the patterns thrown at them on the hardest.
I've tried a number (not hundreds, but probably more than thirty or forty, heh) of bullet hells or otherwise high-ish difficulty shmups over the years, most of it before I figured out how to slow them down manually. I'm well aware of what would and wouldn't surprise me at this point
But yeah, having a slowdown option in game is great. Not enough of the things do that, and there's precious few of those that do that give you enough granularity to really adjust it to fit everyone. Ideally they'd all have an option that bottoms out at Redshift/simultaneous movement and sliders its way up to TAS-only, but until then third party programs open the way