- Edited
This is to free up vertical screen real-estate. Since tabs are already in the sidebar, there's no need to duplicate them at the top! The feature can manifest itself in two ways:
1) Only the address bar stays at the top in "compact mode" with no tabs underneath (note: I realize this is sort of a "hidden feature" already if you turn on the sidebar and then switch to "vertical tabs")
2) Everything disappears, and the address bar compresses itself into the sidebar (similar to Arc)
The user would then be able to use the sidebar exclusively with vertical tabs, rather than having a combination of both.
Here is how it currently looks: