No, it's a pure implementation reason, not related to spec. Rusty is obsessed with accounting and he wants the entire description to be present in his CLN db. I think he's thinking about cases where the description is a big invoice document, but too big to fit in the bolt11, so he wants the hash in the bolt11 but the whole thing in the CLN db.

Totally unnecessary in my view but I stopped fighting that fight, seems way more frictionless to get rid of the lnurl requirement. You'll see a few people disagreeing with me at the bottom of that PR though.