In addition to what others have said, I prefer more vertical space for webpages. Vertical tabs take up much less space and are, in my opinion, much easier to organize. I also don’t need to see the title bar constant and the favicon is plenty for me to keep track of what’s there.

Grouping helps me keep ideas together. I don’t like to bookmark things I’m only going to need for a few hours/days, so grouping tabs helps me keep them open without them getting in the way.