Demultiplexers do effectively the opposite of multiplexers. For a multiplexer, one of several input lines is transferred to an output line according to an address (so can work for parallel to serial data transfer). For a demultiplexer, it's the opposite --information on one input line can be transferred to any of several output lines according to an address. This could be used to convert serial to parallel data, for example. Or you might want to route data to one of multiple outputs (say, to one of several users).