For myself, peace of mind. Previously I installed them over an existing system, never had any issues.

I have a repo for my dotfiles and install scripts so i can set up my system in like an hour

Any tips to setting up macOS dotfiles? I set up a new Mac today and was going to go that route but after trying to do a diff of "defaults read" and seeing that its thousands of lines largely unrelated to macOS I gave up and changed the obvious things using the settings menu and now just have to deal with all the other minor differences as they popup.

I can't help with that sorry, I keep the Mac settings on default as they are usually works for me, dotfiles only have terminal/vim/ssh/git settings :|

I find it quite nice, personally. Reinstalling yearly-ish allows me to examine what tools I'm using and see if I could replace/remove any of them.

This is the bigger thing, really, because the way macOS upgrades is basically to apply your existing setup over a new install.