The advantage of diplexer

The diplexer, being a passive device, is reciprocal: the device itself doesn't have a notion of input or output.

The diplexer is a different device than a passive combiner or splitter. The ports of a diplexer are frequency selective; the ports of a combiner are not. There is also a power "loss" difference - a combiner takes all the power delivered to the S port and equally divides it between the A and B ports. A diplexer does not.

The chief advantage of a diplexer is that it allows two different devices to share a common communications channel. Typically the shared channel is a long piece of coaxial cable. Rather than run two separate cables, a single cable with diplexers at each end is used. The plan is economical if the diplexers cost less than running the second cable.

Diplexers might be used to connect two receivers inside a building to two antennas that are some distance away using a single cable. Alternatively, two transmitters might connect to two antennas.

A diplexer may be used as a form of duplexer, which is a device to enable bi-directional (duplex) communication over a single path. In this usage the high and low frequency signals are travelling in opposite directions at the shared port of the diplexer.