Why would some special mode be needed? I use ctrl+ to get a font size I like, then I set my browser window to a size where the text reflows to what I like. HTML is made to make it easy for the reader. And equally easy on mobile (phone, tablet)
Yeah, sometimes I use that for a site that is otherwise well formatted but the font is just small. For sites I visit often I just save that as a site specific preference so I do agree with you in some cases.
However, with reader mode it goes straight to my preferred font size, has nice margins, and basically everything is formatted in a very readable way. It still reflows and changes width with the window, but the rest of the formatting is better as well. I really hate text that goes right to the edges of the windows with no margins. Just a personal preference, but for me the quickest path to dramatically improved readability on some sites is reader mode.
Tbf, I HAVE encountered sites where zooming in makes it even less readable/prevents zooming/adjusts the font size to be roughly the same when you zoom in.
I wish I could upvote you a thousand times. I hate it when pages have a fixed width -- that's what reflow is for. Let ME choose how I want to read the content.