Progress indication among others. But this is actually the only one retained by current simplified, slimmed and autohiding scrollbars.

Here are others:

-Discoverability: Is there more content? (Scrollbar enabled/disabled) How much? (Size of scrollbar)

-Jumping to a location in the content

-Skimming by hyperfast scrolling

-Coasting by holding a scroll button pressed (this one is forever lost because the buttons have been abolished

-Providing a space for bookmark indicators/in-page search result indicators

There are probably other use cases too.