1) Shows the text even if a modal has already tried to obscure it.
2) Ensures a legible font and contrast.
3) Undoes stupid scroll jacking. Yay, the text scrolls smoothly again.
4) Fixes the scroll bar so that it provides an accurate indication of how far through the page I've read, since the non-text crap I don't care about isn't part of the view now.
5) Allows me to copy text w/o worrying about anything else getting inserted to the clipboard.
6) Doesn't disclose to the site how far I've scrolled, which is none of their business.
And probably other good things I'm forgetting. Tapping the reader view icon as soon as the page loads has become habitual.
I obviously run an ad blocker as well.
This is in both iOS and macOS. Reader view FTW. It's the next best thing to disabling Javascript entirely.
Chrome and Firefox should include an as good reader view built in.