Original: One use for multiplexers is cost savings by connecting a multiplexer and a demultiplexer (or demux) together over a single channel (by connecting the multiplexer's single output to the demultiplexer's single input).
Simplified: One cost-saving use for multiplexers is connecting a multiplexer and a demultiplexer (or demux) together (called mul-dex) over a single communication channel (by connecting the multiplexer's single output to the demultiplexer's single input).