from my personal experience, manipless runs are really fun when approaching a new game.
you can just grab the beginner guide and hop into your first run, no days/weeks spent preparing and learning manips.
all your first runs will pb, because you improve quickly, and each of them feels unique and rewarding, no matter how much bullshit rng/bad stats you get.
for example i recently got into gen7 speedruns, and in my last run i had a negative 0defense/5hp ivs starter: it was a super fun, i had to adapt and improvise in a lot of fights, and i still ended up pbing.
but lets be real, once runs become more optimized, such a bad starter would be unrunnable, all the "adapt and improvise" situations would just be timelosses, and the chanche of pb become close to zero.
thats where manipped runs shine in my opinion, better quality of life at higher levels. i run a lot of red and its already really bad like it is, im not sure if i want to know what it would be if we were still dealing with unmanipped mt.moon, nidoran encounter/stats and so on.
manips remove some rng and add a level of skill that i personally enjoy a lot: hitting triple extended feels rewarding, way more than idk getting through mt.moon manipless with less than x encounter.
learning manips is also fun but maybe im in the minority with this opinion.