If you mostly use your browser in a maximized format, then you'll have empty space to the left and right of most webpages. Vertical tabs make good use of that.
If you tile your browser window often or if you have relatively few tabs (per browser window), then it's not nearly as useful.
I usually make a bookmark when I need a tab later. My bookmarks are organized. I only keep a tab open when I immediately need to use it. Like one tab of the paper I'm reading, one tab Deezer music, one tab search engine, and 1-2 tabs of sites I haven't closed yet but I'm about to.