Here is an example. Without an instrument definition, to call up a particular sound on a Yamaha synth in a MIDI track, I would have to get out the manual, look up the sound, do some calculations with the most significant and least significant bit to get a bank number, look up the patch number, and put this data in the event list. I may also have to tell Sonar which bank select method to use. I also have to know whether to use 0-127 or 1-128 as the patch number and make adjustments as needed.
With instrument definitions, I can open the event list, and choose the instrument from a drop-down list.
I have several external synths I still use and some have expansion cards, so this could be a lot of trouble without the definitions.