Last time I tried Vivaldi, it only had one dimension for the vertical tabs - middle click on a link and it just gets added to the list of tabs.

With TST, I get a second dimension - child tabs are indented in underneath their parent, so that it is clear where they come from. This is the reason that it is called a Tree.

Yea, there is one more level of nesting, you can group tabs but it is a manual effort. I mostly use that when I want to collapse a bunch of pages relevant to a single project that I'm not working on. What problem does the knowing where the tab came from solve?