I don't currently use Signal, but I'm curious if this is a new feature only when starting fresh on a new phone?
I'm guessing if you got a new phone and restored from your old phone backup that it would have included your old chat history automatically without going through this process?
I uninstalled Signal and reinstalled it yesterday on my iPhone and was prompted with this (on the same phone I'd been using it on for months).
Would be more useful to me to have a cloud backup like WhatsApp does (as long as it was encrypted). But as some posters here mentioned, the whole point is security so that's just another link in the chain.