As ddingus implied, in the C64 era you'd actually have state-change logic executed as the beam moved across the screen. For instance, the VIC graphics chip only had support for eight hardware sprites at a time. But by using the hsync interrupt you could reuse those sprites and draw each at different coordinates as long as they were in different regions of the screen, vertically.

[1] https://en.wikipedia.org/wiki/Raster_interrupt