Neither the back button nor bookmarks retain state sufficiently or have sufficient responsiveness to replace open tabs.
I have 2-3 tabs open most of the time. If I am looking something up, I go to 10-12. When I am done, it goes back down to 2-3. I can't even fathom under what situation people are using hundreds of tabs.
I have heaps of tabs open, but there are orders of magnitude more pages in my browser history. The latter usually contains so much that is too much to look through to find anything but quite recent entries. The tabs still open are pages still with some relevance to me, while the history is filled with lots of incidental stuff that is no longer of use.
[Edit: also, with open tabs the page content is already there so it's more quickly accessible. This can be useful if you need to look at it to help find the page you're looking for, and to get the content from the page]
For me, it reduces cognitive load. I don't keep in mind all tabs that I have open -- that's why I keep them open! To forget about them.
Bookmarks are mentally for things I want to save and reuse. But I'll probably open 40 HN sourced articles (from the main page and comments) per day, and continually cycle/review them for interest/effort throughout the day.
I also close tabs as soon as I'm done with them which is why I only have 150-200 tabs open. Each categorized by activity, so when I switch one activity to the other I save time and cognitive load by having them right as I left them instead of having to find what URLs to reopen and actually open them. This way I only have 5-30 tabs open in front of me while the other are offscreen in other groups.
If browsers had a sort of tab queue where longer lived stuff then things could be a bit better on this front I think