If your talking putting sand in a speaker stands its for added weight for more stability. If your talking INSIDE of a speaker, It may be for added stability or to deaden the panels. I really couldnt tell you. To really get the scoop on this you would have to talk to the person who designed it and what their particular intentions were.
For the most part, Keep the sand on the beach were it belongs.
I heard of one guy who dumped sand in his preamp thinking it would make it less resonant or whatever and he ended up getting it all in his switches and volume control...