It's just a way to communicate to a specific device for features that may not be covered in control changes.
For example if you have an guitar multi-effect pedal you could change the delay on the reverb .. or change the VCF on a synth, etc.
It's the next step deeper into MIDI control after control changes.
However, that hexadecimal aspect throws most people.
It's really not necessary to understand the hex, you just need to know which numbers to change to get a desired effect. Usually the MIDI documentation in the owners manual will give this information.

I don't think BiaB handles sysex this way, but RB and PT will.
Some of my older synths will send the sysex message over MIDI to be recorded by the DAW in realtime if any contoller on the synth is moved.

Most practical use is to backup FX units. Quite a few will allow a sysex dump that holds all the needed information to do a backup of memory settings. Saved my behind a couple times when I had guitar FX set up for a show and the battery that allowed the unit to store all the fine-tuned FX settings died. When the battery was replaced it reverted back to factory settings. I just set it up to receive a dump and played it back into memory from PTPA via MIDI.

Edited by rharv (01/21/12 12:29 PM)
Make your sound your own!