I was trying to find an old article that I think was in Sound on Sound but a lot of the older stuff is not online. It covered some techniques for enhancing MIDI parts by creating duplicate tracks and then duplicating some or all of the notes.
 
I think a lot of what they suggested is now pretty mainstream like adding a sub bass note, like a simple sine, to some of the bass drum beats or to selected notes from the bass line. Mixed in so that it is felt rather than heard.
 
Another example was to take the right hand part of a piano and duplicate it to a marimba an octave or two higher, set the volume until you can just hear it then turn it down slightly. If available the suggestion was that you added some reverb to the marimba to create some high end gloss.
 
From memory a lot of the suggestions covered adding bottom end, mid range punch or high end gloss by adding some additional MIDI instrumentation based on an existing part pitched to fill the gap. And this was in the days when the processing and memory capabilities of MIDI sound sources was pretty limited. Not sure that these techniques made the sounds more realistic but executed with care they did make the overall sound better,
 
It may have been something like this or the layering of MIDI sounds that was being suggested.
 
Of course these days there are easier ways of achieving some of these effects.

All clever thoughts. You can't break it. Have a party.

I love midi. Synths are terrific fun.

ooblecaboodle

dubdisciple
It wasn't obvious to me at all

reading comprehension not strong point for you is?

On the opposite.
Comprehending the OP the only correct interpretation would be the one Dubdisciple made. The OP clearly says: I haven't used exciters on MIDI tracks yet. He doesn't mention audio at all. 
Then again, it's quite possible that he meant audio tracks, but that we do not know. Just reading the text, it's MIDI he talks about.

midi is midi.
 
audio is audio.
 
aural exciters are for audio.
it's in the name.
 

We all know he meant on the output of a midi synth. He wasn't trying to apply an exciter to the midi data itself.

I feel with care in your engineering process you don't need an aural exciter.  I agree with Sharke in that using it to give some top end life to dull sounding sources e.g. transferring early recordings etc can be useful.  But in productions where you have full control from start to end I have found anyway you don't need it.  I am usually trying to tame top end most of the time, not increase it.

dubdisciple
I can't even imagine how a midi exciter would work. A few audio effects can be done more or less with midi ( delay being the obvious choice), but an exciter would be a ridiculous resource hog and sound terrible if attempted via midi.

As Frank Zappa would probably put it, aurally exciting MIDI is a bit like fishing about architecture!

ooblecaboodle

dubdisciple
It wasn't obvious to me at all

reading comprehension not strong point for you is?

Being an a-hole is obviously a strongpoint for you

Jeff Evans
I feel with care in your engineering process you don't need an aural exciter.  

The same is true of eq, compressors, chorus, delay and reverb. But it's nice to have those tools to shape sounds into whatever way you so desire.

I once tried a MIDI track with no audio. It was pretty boring. Then I turned on the amplifier.
 
It's common to refer to software instrument tracks as "MIDI tracks", just to distinguish them from pure audio tracks. You might say "this project has 8 audio tracks and 4 MIDI tracks". Even if a screenshot clearly showed 16 tracks in the project.
 
To address the original question: an exciter is a distortion unit. It adds harmonic distortion much the same way a fuzzbox does, just more gently.
 
Using exciters is like using salt in cooking: a little goes a long way, and the effect is cumulative. You can end up with salty-tasting soup even though you followed the recipe, if you didn't take into account the saltiness of the bacon you added. Similarly, distortion is additive. Like salty ingredients, one adds zest but multiples will cumulatively result in too much spice. Too many exciters results in nasty, grainy-sounding mixes.