I mainly use them to import things into musicbrainz but I'm wondering how else these are used to improve the browsing experience across the web?

I'm using ublock and consent-o-matic. Former against ads, later against cookie banners

For YouTube specifically:

I made one myself because there were about 50 things pissing me off about YouTube and YouTube Music: https://github.com/Sv443/BetterYTM

I also made another one because my YouTube embeds kept being blocked as I was using a VPN, to allow me to open them in a new tab instead: https://github.com/Sv443/YouTube-Embed-Links

Other than that I'm only really using regular browser extensions like uBO and the likes.

Same here. I feel userscripts are incredibly powerful tools to make things better in some ways. Nowadays I don't really use any of them anymore. The ones I did use for a time was Enhanced Steam Gifts.

There was also one for Steam, but it eventually became a separate browser extension: Augmented Steam.

Lemmy Universal Link Switcher, Linkify Plus Plus and Mouseover Popup Image Viewer are all very useful.