The advantage of fixed-event or fixed-time leveling is that players can plan for it reasonably. When its "whenever the GM decides" we're back to players often not really having a clue.
I've been playing D&D for 30 years and the idea that players "plan for leveling" outside of power-gamers who have a spreadsheet listing every ability they're intending to take for the next 20 levels seems pretty hilariously nonsensical to me, I have to say.
Really, we used XP-based leveling up to the end of 3E, and no-one except power-gamers was "planning" for their level up. In no D&D group I played in. After 3E, in 4E and 5E, I've played in groups using milestone (so fixed-event) and DM chooses and even one using XP, and in none of these have players been "reasonably planning" for their level-ups, again, except power-gamers. Some players have some vague idea what they want, some have a precise one, but the idea that knowing approximately when they're going to level helps with this? That just seems profoundly theoretical and nonsensical. YMMV of course but I am really pulling a very skeptical face here.
In addition, it hasn't actually mattered in 4E or 5E.
Why? Because literally all the 4E and 5E groups I've played in have been using digital stuff (except at the very beginning of each edition), so people just level up characters with the click of a button, and are presented with the choices. In 5E, I don't think it would matter even if it wasn't digital, because your choices are so narrow.
I do admit that if I was playing 3.XE, PF1, or 4E, and I wasn't using any digital stuff (and I guess with 4E you can't now?), then it might be slightly annoying, but, that's
so last century.