Back in the day, most cool graphical effects were done by using the CPU to update video memory at the precisely timed moments during the rendering of a frame. For example, if you changed the "scrolling speed" of a background between the drawing of every line, you could get a nifty "parallax scrolling" effect out of a machine with only 1 background layer. Or, you could change a color palette halfway through the screen to make it look like that part was underwater.
Some systems had interrupts that would fire in-between the drawing of lines to facilitate this. Other, simpler ones, like the NES and the C64, basically required the programmer to write perfectly timed, cycle accurate code that would poke the right values into video registers within a very small timing window.
The Amiga had a number of custom chipsets to make it easier to produce effects like this, and free the CPU from spinning idly and poking registers all the time. Looking at the wikipedia summary of its capabilities, I imagine console game programmers of the time would have killed for the ability to say "please write this value here at this exact moment, thank you."
It reminds me a bit of the SNES's HDMA (horizontal interrupt DMA) function, which allowed programmers to specify tables of data to be copied to certain VRAM registers in between the rendering of scanlines. Pretty much every SNES game with cool "mode 7" effects relied heavily on it.