When you're keeping things in a tree structure for visual grouping and using containers to manage different logins, bookmarks will lose the tree structure, and you'll have to specify which container to open it in. If your workflow involves a dozen tabs per context, locating the bookmarks and reopening them every time you switch contexts is a significant time and productivity loss.

Consider the classic Evidence Board (also known as string wall, crazy wall, conspiracy board, etc.). Saving everything to bookmarks is the equivalent of putting your board's contents into a drawer, then pinning everything back up whenever you need to look at or update that particular conspiracy. It works, but it's cumbersome, error-prone, and wastes a lot of time; you'd only do this if you only have one board but multiple things to inspect. Leaving tabs open and simply unloading the inactive tab trees is like having multiple separate boards where you just roll them into a closet when you aren't using them.

Personally, I can't use bookmarks because if they're out of sight, they get forgotten. Keeping things in an open tab is like having the browser constantly bugging me to remind me that I have to do this thing. It doesn't guarantee that it gets addressed in a timely manner, but with the alternative it's guaranteed to not be done at all.

It also helps to keep my place in my work. There's things that I'll always have open because I need quick access to them and don't want the friction of trying to find the page to lead to procrastination. Same with anything that's relevant to work in progress.