One example of using a serdes is when sending several low bit-rate channels into a single high bit-rate channel for example on a fiber optic in this case what u need in the transmitter is a serializer to multiplex the low bit-rate channels into a high bit-rate signal that is sent through the fiber optic overseas and at the other end what u need is de-serializer to demultiplex the high bit-rate signal back to the low bit-rate signals
It is basically used to replace the old parallel buses. So you take all the parallel data and serialized them. Then you transmit them serially. On the other side, the serial data is then recovered using the CDR (clock data recovery). Usually some type of coding(8b/10b) is used for serial data transfer. Using a serdes for data transfer is advantageous as the number of lanes are reduced and the routing gets easier. Most of the time the data is transmitted differentially so the inherent noise cancellation helps too.
Example:
PCIE express - for Personal computer peripheral communications.
Rambus Raser: Networking.
Hyper transport: For high speed chip to chip communication.