Finding things in Finder · Storage Studio in Finder's sidebar
Why did the Storage Studio location disappear from Finder when I quit the app?
Short answer: Quitting Storage Studio really stops it, so its Finder location is removed and background activity ends. Reopen the app and it returns; turn on Launch Storage Studio at login to keep it there.
Because quitting Storage Studio really stops it. When you quit, the Storage Studio location is removed from Finder and all background activity ends — nothing keeps running on your behalf.
Open Storage Studio again and the location comes back with your accounts as before. If you want it there whenever you're signed in, turn on Launch Storage Studio at login in Settings → Advanced.
Files you had open from it should be saved before quitting, as the location goes away with the app.