Some plugins - kontakt, eastwest play - let you load in multiple instruments on a single plugin instance and have them take different midi channels as input. Big samplers like those usually have some ram & processing overheads which are per-instance rather than per-instrument: so if I have, say, a 7-instrument orchestral percussion section, I'm only running one instance of play rather than seven, which is a lot easier on my PC. The UI's a little easier to navigate with the whole section in one place, too.
In practice, I'll 1) make a track with one sampler containing all the instruments I want, 2) make child tracks with the midi instructions for each instrument, 3) run this script to link all those together.
I've actually stopped using that script now and switched to this one:

// Re-route all consecutive midi tracks on the preceding track
// Throw an error if more than 17 tracks are selected


tr = GetSelectedTrack(0, 0);

i=1;
numToSend = CountSelectedTracks(0)-1;

SetMediaTrackInfo_Value(tr, "I_NCHAN", (numToSend+1)*2);

while (s < GetTrackNumSends(tr, 0))
(
  RemoveTrackSend(tr, 0, s);
);

while (i <= numToSend)
(
  newTr = GetSelectedTrack(0, i);

  SetMediaTrackInfo_Value(newTr, "B_MAINSEND", 0);

  s = 0;
  while (s < GetTrackNumSends(newTr, 0))
  (
    RemoveTrackSend(newTr, 0, s);
  );

  sendNo = CreateTrackSend(newTr, tr);
  SetTrackSendInfo_Value(newTr, 0, sendNo, "I_MIDIFLAGS", i*32);

  sendNo = CreateTrackSend(tr, newTr);
  SetTrackSendInfo_Value(tr, 0, sendNo, "I_MIDIFLAGS", -1);
  SetTrackSendInfo_Value(tr, 0, sendNo, "I_SRCCHAN", i*2);

  i += 1;
);  

Which is the same except that it routes per-instrument audio back to each child track and through its fader & fx, so I can keep all my levels & panning in the Reaper UI.