In programation, the raster is often referred as the act to modify the setting of inks between 2 scanlines, in interrupt mode.
This is a way to display more than the usual Video modes number of colours.
The time between the display of 2 Scanlines by the CRTC allow the CPU to order the Gate array to modify the setting of inks.
The screen display at 50Hz...The CPU (Z80) runs at about 4 MgHz.
So It has enough cycles between 2 scanlines to order this.
You can also use Split raster to "actually" display 2 different screens, sometimes with different Video Modes. This also allows a Graphic heavy HUD with no interference with Scrolling.