When drawing animated images directly on the screen, many times an undesirable effect named flickering is perceived.
A common technique to avoid this is to draw images on an off-screen buffer, and them copy its contents to the screen when the drawing operations are finished. To implement this technique two buffers are required, hence the term "double buffering". This off-screen buffer has the same dimensions of the screen.
During the time of MIDP 1.0 phones, there was no Java support for this technique, so the developer had to implement it.
However, for MIDP 2.0 phones, the GameCanvas API supports double-buffering automatically. Here you can find a sample of this technique.