The omnibar feature also searches your bookmarks and history, so they are recoverable using the same method if you closed the tab.
But that depends on you thinking to bookmark the page; or accepting that some things will age out of the browser history. So I guess it's kind of like a fragile bookmark: probably reliable, but not the end of the world if tab restoration failed for some reason.
Also, (I assume) tabs retain their Previous Page list, which also has some value.
The tab will also have the context around it (in form of other tabs). When you often switch between various tech stacks/frameworks and open 30 pages of documentation for each one, it's useful to be able to restore all of it easily without messing with bookmarks (that haven't improved in the slightest over the past two decades).
I haven't regularly bookmarked sites in a decade or more. The bookmarks are in my head, and I can live with the Omnibar mostly reliably surfacing the things I didn't finish, and later recall and want to pick up.
That said, I'm pretty sure the Omnibar is buggy at finding tabs. I inevitably have several tabs to gmail "open". It ought to be a lot easier than it is to find /return to the last-used gmail tab, and not one of the several from previous browsing sessions. :)
The only stuff I tend to bookmark are things I want to use as a keyword search, everything else is a tab in a certain window. There's not enough of them that I have to search to find, maybe a fifty in total?
There is the rare bookmark in my toolbar tabs for undetermined future stuff.
Sometimes they come up in my search results in the Omnibar. Open tabs are in the top rank of results.