There's a UX difference: when you look at your inbox from fresh you have to remember which ones you purposefully ignored because they were left unread in the inbox (this might be trivial for you if you're used to it).

I practice inbox zero also, the value for me is knowing that if it's in my inbox it's because it requires actioning, if it's not it's ignored (deleted or archived).

I also just generally like deleting things as much as possible, I don't like the cruft. If I have to search through old emails I don't have to filter by stars or anything like that, I like knowing that if it exists it's because it's important.