It's especially good for apartment searches and job hunting. Companies that try to link buyer and seller are naturally spammy, so you create a new one for each contact. Then when you find one, you delete the others.
It's also makes it dead easy to filter all that information into folders, so you have a neat record of all your interactions with the various companies you're dealing with.
Most importantly: once you find a job / landlord / whatever, you really want a reliable line of communication. This way, your address with them never changes, but all the spam goes into the bit bucket.
Also, it's useful for have i been pwned. One of my few accounts which uses a grouped address is in today's data dump, and it wasn't clear what account it was, because of the grouping.
It’s awesome for filtering and sorting emails though.