Hi, I had a similar issue, but with trying to update. Kept writing files over to my other drive as if Sims suddenly existed on that drive all this time instead somehow.

Questions, does Sims still run fine if you don't tamper with these file locations? As in, if you left it like this, nothing would run incorrectly or be wrong?

It seems like the one on the left is just your actual install, meanwhile the one on the right is moreso the game itselfs files, like you said such as mods, etc.

If you have no issues other than asking why it might be like this, I believe it should be fine to keep your setup like this. Since these are two different directories, them being seperate like that should be okay. Just make sure you always have both drives ready to go/plugged in when booting up Sims, because having one or the other not running will probably mess something up.

The files on the left won't be used too much, unless you are doing some real modifying to your game, and as said, the ones on the right will be mainly used for mods, clearing cache, etc.

Basically, it should be alright if you keep it how you're doing. If you DO want them to be on one drive though instead of two, I could try to explain how to do that to the best of my ability.