I use TreeStyleTab, and for me it's a mixture.
For example, I have a bunch of tabs for online grocery shops in recent months. These are for reference. Whenever I'm in the mood to check for delivery slots (which have become scarce), I click through each of them, login and check if there's a viable delivery slot available. The set of tabs is a reference to remind me of the list of 10 or so shops to check periodically. I might forget some of them otherwise, because they are not all ones I use regularly; they are only in the list to improve my chances when slots become hard to obtain.
Then I'll use the same tabs for convenience rather than reference. Muscle memory and visual location. If I found a slot, I might place an order in stages over a few days (one to book the slot, then adding items over the coming days if I realise I need something, finally to amend the order to fit my needs shortly before the delivery date). Then it's useful to have the tabs in use in a convenient location. Their visibility as a pinned tab also serves as a reminder until the shopping is finished (typically ~3-12 days) in case I forget to amend the order.
I have a similar thing going on with financial institutions. I have about 20 of these in use, counting both personal and business use (I run accounts of two businesses), banks, credit cards, and tax pages. From time to time I'll want to peruse them and check in on balances and pending tasks, occasionally download a statement. Then it's useful to have a list in one place, similar to with shops. without a list I'd probably miss one.
Both of the above could in theory be replaced by bookmarks, but there's a bunch of UX reasons why bookmarks don't work so well. One of them is that we really need a "live bookmarks folder" for each group. If I open another shop/institution tab, I need that to remain in the "persistent" set by default. Using a normal bookmark group, I'd open the group as tabs, then half an hour after I open something else, I'd have to remember to add that extra item to the bookmark group, and just that item. But after an hour of browsing and having visited maybe 250 pages, I'm likely to forget which open tabs are already in a bookmark group and which one's aren't. It's too much work to step through and compare item by item by hand when I'm done and need to move to another task, so I end up taking new "snapshot" bookmark groups rather than managing the existing bookmark set. This gets messy fast. In practice it's easier to just keep a set of tabs open, and delete the ones which are no longer required.
Also in my tabs are the articles (many from HN) that I collect into a knowledge base. Tabs aren't a great way to save these, but neither are bookmarks. Ideally I'd want something that saves them with tags and brief notes. For now, I send tabs from my active window over to a dedicate HN window, where they form a long queue of things to go through in a systematic batch at some point and group, sort, categorise, filter, and take notes on the salient points I want proper notes on. They are a kind of research multi-queue. You could think of them as a set of about 50 long-term tasks that are likely to take years to produce the results I want (which are for actual tech developments, not reminiscence). From time to time I'm tempted to batch convert these to bookmarks, but doing so makes them unusable because it loses scroll position and text highlight, so it's labour intensive having to take notes instead. I guess people use "web clipper" software for this kind of task. I haven't found something I'm happy with yet, but I haven't tried much either. I wouldn't be happy locking so much long term research into proprietary software though. On my phone, I use "Share to Orgzly" to capture such things, rather than tabs.