One DMX channel provides 8-bit digital resolution, which is 256 steps.  Ganging two channels together provides 16-bit resolution, which is 65,536 steps and is much smoother when dimming LEDs.  You can think of this as coarse and fine channels controlling a single load, where the fine channel provides 256 levels within one value of the coarse channel.  The RC4 16-bit raw dimmer curve provides this functionality using two consecutive channels in the DMX universe.  An option is provided to use the channels in either order.