Rates, for those who don't know, are speed modifiers such as 1.1x, 0.9x 1.3x etc. These allow the player to play a song (Say a map you like but arent good enough to play at fullspeed, but Half Time is way too easy) at their own custom speed to suit their needs.
The reason I think osu! needs this is because the HalfTime DoubleTime system is ancient and flawed. You can already see the effects of this terrible system in modes like mania, where with the addition of DT PP, it's go fast or stop trying to play the game anymore. However to fix this issue, Rates can be added, which allows accuracy players to still get good scores at 1.1, 1.2 etc. and speed players can push themselves with 1.6x and beyond.
This can also apply to other gamemodes, keep in mind I do not claim to be as experienced in other gamemodes, but I'll try to explain it anyway. Songs in standard have basically reached this point where they require ridiculous PP-Abuse to even come close to top players ranks (With the exception of a few). So with the addition of rates, songs that aren't pp abuse (Lets pretend this is Rafis/Gayz/Cookiezi on FREEDOM DiVE) can be put on 1.1x, 1.2x etc. So that the player isnt forced to play on DT where the song would be near-impossible, but at 1.1x where the song is pushing a new limit. TL;DR: Varies top scores so that the top scores arent the same maps for nearly everyone. Also allows for new ridiculous plays.
As for Taiko and CTB (is it called osu!catch now?) I really don't play them enough to talk on them, since I'll just look like an idiot to the actual gamemode players. But if any taiko/CTB players want to voice their opinions on this PLEASE DO. It would help me a great deal.
"This isn't Stepmania."
So what if another game had it first? Infact stepmania wasnt even the first game to do it! It's been a staple of VSRGs for longer than osu! was even a thing.
"Too many options would confuse new players."
At the moment I think the HalfTime, DoubleTime system would actually confuse players more, since HalfTime isnt even half, its' 0.75x, and Double time isnt double, it's 1.5x which I think is even more confusing than a system that clearly shows how the song is being altered.
Mania is broke by the lack of rates, fix it. Other gamemodes can benefit too.