The mirrorlist package gets updated when there are changes to the mirrorlist. For example, new mirrors have been added, defunct mirrors have been removed, a URL has changed, etc. You don’t necessarily need to have the most recent changes in your file, but you are less likely to run into issues if your mirrorlist is current.
That being said, you don’t actually need the mirrorlist file as it is provided from the repo if you are using a tool like rate-mirrors to sort the mirrorlist. The mirrorlist generated by rate-mirrors will always be better because it is sorted according to mirror availability and latency, where the mirrorlist from the repo (the pacnew file) is not going to be sorted in a meaningful way.
When you run garuda-update, it automatically runs rate-mirrors for you. It fetches mirrors from the mirror status page (Arch Linux - Mirror Status) and builds the list from scratch according to what mirrors perform best for you. In this case the pacnew from the repo is entirely unneeded.
All that to say: if you are refreshing your mirrorlist with garuda-update or rate-mirrors, you can just delete the mirrorlist pacnew files whenever they come down because you don’t need them.
sudo rm /etc/pacman.d/mirrorlist.pacnew