I first heard of that in context of writing to the screen, actually, when you don't want to redraw all the things every frame, or simply can't afford to, and only draw the things that have changed (i.e. are "dirty"). So if you were to keep track of the old position of an object, you might redraw the area of background where that object was, and then just draw the object at the new position, instead of drawing the background for the whole screen, and then the object.
Browsers are also kinda heavy on this when it comes to both layout and drawing. E.g. when the content of a div with fixed size changes, you only have to layout and draw the text in the div -- but if the size isn't fixed, you might relayout and redraw everything on the page that follows the div, but might not need to touch anything that comes before it. I'm sure it's incredibly complex, but still much faster than not doing it.
I first heard of that in context of writing to the screen, actually, when you don't want to redraw all the things every frame, or simply can't afford to, and only draw the things that have changed (i.e. are "dirty"). So if you were to keep track of the old position of an object, you might redraw the area of background where that object was, and then just draw the object at the new position, instead of drawing the background for the whole screen, and then the object.
Browsers are also kinda heavy on this when it comes to both layout and drawing. E.g. when the content of a div with fixed size changes, you only have to layout and draw the text in the div -- but if the size isn't fixed, you might relayout and redraw everything on the page that follows the div, but might not need to touch anything that comes before it. I'm sure it's incredibly complex, but still much faster than not doing it.