1. Great if you have a wider screen (could never do it on my old 13" Macbook Air, for a 15" it's pretty good but for a 24" iMac it's perfect). But if you need the space youjust have it set to minimize by default, maximize on hover.
2. See the titles of your browser tabs, which is great when you are like me and never have fewer than 30 tabs open at once.
3. Easier to select browser tabs when you have many of them open (ie they don't get squished unreadably small)
I still use horizontal tabs, but have dabbled with vertical tabs. The biggest benefit I saw was that tab names stayed a consistent width and readable, no matter now many tabs I had open. With horizontal tabs, once you have over 10-15, you’re kind of flying blind.
Basically, it's easier to see tab titles when you have a LOT of tabs open.
They really only work if you have a large monitor. I use a 50" 4k TV and two other monitors, and a 15" laptop. When I'm on smaller screens I have to hide the tabs.
Your typical screen today is wider than most content on the web needs it to be, so most web pages render as tall columns with ample margins. Vertical tabs use up that space efficiently to show a large number of tabs in a way that makes it much easier to see all your tabs at a glance without losing track because the titles are shortened into nothing.
