First hand testimony from someone who shall remain anonymous unless they chose to reveal themselves:
I have >1k in [my gitea instance]
1k is for various reasons, when I am researching how others have done a specific thing, I usually create a mirror for it in case it disappears so I still view it if it disappears. Probably 50% of what I’ve mirrored is now gone(so truthfully only 500mirrors, as when I get an alert they are gone I convert the repo). Also repos they are possibly going to get a dmca, such as YouTube-dl I like to have a copy of.
I also have a script that mirrors entire users/orgs from GitHub
Mirroring things has saved me more than once.