If you habitually have a lot of tabs open, you'll probably know how annoying it is finding things when each page title has been condensed down to 4-5 characters. On widescreen displays (especially 16:9), vertical pixels are also a lot more precious, while horizontal ones are plentiful.
For me (3840×2160 display, 200% scale), its vertical tab sidebar fits about 30 tabs before needing a scrollbar, and you get a full width title for each and every one.
It can be a bit of an adjustment at first, but I've been using this since the pre-WebExtensions days (since around Firefox 4.0), it's definitely one of my must-haves.