The extreme low end of graphic resolutions in VGA mode. 320x240, while quite blazingly fast, makes the pixels far too large to work or play in. Granted, some old games are 320x240 or nothing, but for my money, I much prefer 640x480 or 800x600 resolution.

A 320x240x256 resolution, unlike 320x200, provides square pixels. With pixels this big, it's important.

Unfortunately it requires more than 64k of ram, so mode X (as this was sometimes called) required the pixels to be separated into bit planes. (In DOS/real mode there was only a 64k block of memory addresses alotted to the VGA card.) A single pixel is arranged like this: (shown compared to 320x200)

       Mode X                      Mode 13
  /___
 /0/4/____   plane 0
^^^^^^^^^^                      (only one plane)
  /___
 /1/5/____   plane 1           /_______________
^^^^^^^^^^                    /0/1/2/3/4/5/6/7/___
  /___                       ^^^^^^^^^^^^^^^^^^^^^
 /2/6/____   plane 2
^^^^^^^^^^
  /___
 /3/7/____   plane 3
^^^^^^^^^^

But yeesh... that's pretty messy. Is it any wonder no one uses bit planes anymore?

On newer, VESA compliant video cards, more than 64k is allowed through windowing in real mode, and in protected mode there is no 64k limit.