Tab cram. If you have a lot of tabs horizontally, they shrink/scroll and you can't read any captions on them, just icons in the best case. But having tabs vertically helps a lot.

Another reason would be that you can have tabs for your tabs, essentially. If you've used something like Sideberry for Firefox (my personal favourite), it has a workspace feature, which means that you can toggle between lists of tabs in your browser.

I'm not that bad of a tab hoarder, but vertical space matters much more than horizontal space, at least for my usage pattern, and having tabs on the side also helps.

BTW, Firefox Nightly have just landed native support for vertical tabs that can be toggled in about:config (for now). Set both sidebar.revamp and sidebar.verticalTabs to true in about:config and you're golden.

Edit: my dumb ass referenced Zen browser's vertical tabs under the post about the Zen browser's vertical tabs

Funnily enough, this exists

Lmao, local memelords meme so hard it splashes out to Fediverse. What a weird timeline we live in.