I'd like to give an example on why keeping all of your skills while level synced would be beneficial.
I have been primarily leveling WHM through roulettes, and in my time leveling it during Stormblood I can count on one hand the number of times the roulette has put me through dungeons that are actually relevant to my current level, which means I have had little opportunity to try out and learn what my new level 60+ skills do.
So I bet you can imagine my surprise after I queued up for a leveling roulette expecting yet another ARR dungeon and was put into Bardam's Mettle instead. Of course the one time I want to get one of the early ARR dungeons for the Moogle Tomestones I'm put into a level-relevant dungeon that I never get in literally any other instance but I digress. Granted the party was able to get through the dungeon without too many casualties, it was still a bit stressful because I had to teach myself like 15 levels' worth of skills in such a short amount of time and even then I wasn't sure if I was doing things correctly.
Yes, I could have spammed level-relevant dungeons and gradually learn things that way, but WHM isn't really a class I actively play. My point is it would be nice to have everything available to me so I'm not stuck using all the low-level spells when I'm thrown into Copperhell for the 18th time and never really being able to play with the newer ones.