I have a lot of tabs open. Not multiples of hundreds at the moment, but probably around 100. I use the same computer for work and personal, so I have different contexts I switch through at least once a day. Increasingly, things are becoming web apps, so I have a dozen just to do basic tasks these days: email, multiple chat clients, music player, code repository, issue tracker, Twitter, online office suite, etc. Sure, I could bookmark and close and re-open every time, but that's a waste of time when I want to quickly switch back to something. And not every app has sensible bookmarking semantics.
Then throughout the course of the day I end up looking up API docs, get linked to blog posts, news articles, and YouTube videos, and read articles which themselves have relevant links to follow. Most of these I just open in a background tab to check out later in the day. These accumulate until I have time to go through and quickly review them. Those that I want to read and don't have the time currently go to Pocket. The rest get read or summarily closed out. I find bookmarks to be a terrible way to triage tabs.
This workflow works for me (and evidently others). It's faster than bookmarking. It's less prone to failure, in my experience (I've suffered bookmark corruption more than once). And a modern computer ought to handle many background tabs just fine. Moreover, if browsers aren't expected to be used in this fashion, they really should set an upper-limit on the number of tabs that can be opened.
Hopefully this gives you some perspective on alternative use cases. It sounds like your workflow works out well for you. I've tried it and couldn't get it to stick. If that means I don't know how to use a browser, so be it. At this point, there's enough of us (your grandmother included) that maybe the browser vendors should just find a way to cope with it better.