From Computer History Wiki
Display refresh is the process of re-painting the image on a display. It was absolutely necessary on CRT displays, since most CRTS are not persistent (i.e. the image drawn on their screen by the electron beam quickly fades). Other display technologies use it too, though, for a variety of reasons. For instance, in most bit-mapped displays, the memory in which the image is created is not actually in the hardware which generates the light of the display itself (since that memory needs to be always accessible from the CPU); rather than only update the display's copy of pixels which change, it is simpler to just copy the whole thing over periodically.