A MIDI Merge Unit (merger) has applications wherever you need to combine MIDI datastreams. You may want to
combine several simultaneous performances on the input of a computer or a sound module. You may want a MIDI Merge Unit to run a computer patch-editor to a synth-module at the same time as playing the module from a keyboard. You may want your MIDI merge unit to insert MIDI synchronization (MIDI clocks or Time Code) into a performance datastream.
Sadly, you can't combine MIDI datastreams just by joining the wires together. Data that arrived at the same time would clash - and be corrupted into garbage.
In addition, the 'bytes' of information must be organised so as to be in accord with the MIDI protocol - so merging MIDI data is a job for a microcomputer - a MIDI merge unit or merger.