There used to be a time when the game would render to a buffer slightly larger than the screen, then blit the right portion of that buffer to the screen. This meant that scrolling just consisted in blitting from a different offset. Composing the background image, then, would only require changing the bits behind animated sprites (so you would usually intersect the old and new position of each sprite with the background tile grid to find which tiles to draw again).

All of this changed with the advent of 3d hardware acceleration, but I remember doing it up until 2005 to get significant performance gains (on bad hardware).