They were created specifically for communicating with the dead. There are tons of supposed origins, but most of them are likely apocryphal.

During the spiritualist craze of the 1800's people would have "table tipping" seances where the table would tap out answers. While this worked well enough for yes/no answers, having it spell things out was cumbersome. A system was developed using the "planchette" to point to letters and numbers to make talking to the dead more efficient. The use of the planchette was common enough in 1853 to spur letters condemning them from the clergy.

Check out this site for some interesting history of the boards.