They have a ton of lossless audio, and you generally have choice in terms of which version of something you would like (e.g., an older release, if you don't like the remasters [1], or want specific deluxe/international editions -- Japanese CDs often come with bonus tracks). They also have very helpful curated collections of similar music and such.
Plus, having a RED account tends to help serve as a way to get to other trackers (of course, you should genuinely use the site rather than collecting accounts, etc.). RED is also unique compared to a lot of other decent trackers in that they don't require you to be invited by someone else or already have an account on another site; you can sign up by doing a short interview on their IRC [2]. Worth noting, though, that it'd be best to do it on a Friday night or something when you'd have time to waste; all of these sites are run by volunteers, so it can be a few hours of idling before you'd actually get a chance to sign-up.
For East Asian music, like K-pop, J-pop, J-rock, etc., Jpopsuki has the most extensive collection. SugoiMusic is another one, but I haven't checked it out very much.
And although it is primarily an eBook site, MyAnonamouse ("MAM") has a fair bit of sheet music, plus they also do IRC sign-ups.
Other than that, though, Orpheus ("OPS") is another decent music tracker. But realistically, RED tends to have much more available. However, I do think it's a cool site and I do respect how transparent they are, with even the site being open-source [3].
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[1]: <https://en.wikipedia.org/wiki/Loudness_war> [2]: <https://interviewfor.red/en/index.html> [3]: <https://github.com/OPSnet>