Almost after every shoot, people come back "remember that one photo, where I smiled at sth? I'm very sentimental about that, cause it's [some important thing to them]", which necessitates the need to hold on to every photo taken on the session. So no real deletes here, even if it came out technically wrong (blurry, blown out, etc.).
Those requests lessen, but don't die down completely. Especially with cyclic events, organizers have this habit of a asking for things done exactly year ago.
Some just say: "hey, I remember you taking a photo of me then and then" for their dancing portfolio in my case.
Especially for videos, which can be a constant flow of editing requests, for supercuts and etc.
Now, if I were really smart, I'd just have some good way to archive after two years, and delete after - let's say three years. In practice though, there are so many unforseen circumstances that a habit of "never delete anything" forms really easily.
It's just a lot easier and cheaper to buy another drive instead of culling 10k of photos every once in a while, especially if external confirmation is involved.