Perhaps, but this allows you to do it instantly at will. Like if you only have 8GB of RAM or something (or even worse like 4GB), and RAM is getting tight, you can just right click on your current tab, and press "Unload other tabs", it unloads all but the current tab (except for pinned tabs, you usually need to unload those individually). Like I said not super necessary if you have high RAM like I do now (32GB), but just 2 years ago I still had 8GB, and just 5 years ago I had 4GB (single channel in both cases). This extension actually doesn't automatically do it AFAIK. It's only manual.
Ahhh... yeah... I joined lemmy.world back when the whole Reddit protest was happening. I was using it for a while, but then I kinda just... stopped. I'm still a mod, so I'd log in every once in a while to check on things, but those times have been few and far between. I'd love to say that I intend to start using it a bunch again, but I can't promise that I'll actually do it (I mean my track record is kinda terrible lol). I do like the idea of supporting the fediverse though, so I'll give it one more shot.
I use bookmarks for stuff I may want to access later (as in another day), not stuff I want to access within a few hours. If I used bookmarks for everything my bookmarks would be even more inflated and unorganized than they currently are. I'm not one of those people who keeps hundreds of tabs open for weeks on end, I just have a habit of opening tens of them each day. I usually close my tabs once or twice a day anyway.
Also if you have a low amount of RAM (which there are some that still do), you may not want to only have 2 tabs open at once. Sometimes it's much more convenient to just leave stuff open.
You do have a bit of a point though. I could start using bookmarks instead of pinned tabs for some things, and I guess a good bit of it boils down to me being unorganized.