Can someone explain to me what is the advantage of this compared to the already existing multi-desktop feature?
it offers sub desktop grouping. I too used multiple desktops before this, but with stage manager instead of 4-5 desktops I use 2-3 maximum.
For example I group one desktop for messaging, discord, slack, telegram etc.
One desktop for web browsers and documents, and the other for music or whatever I happen to be doing at that moment. It is way faster than minimizing.