When you are talking about layering a very similar thing applies to layering synth parts as well. For example even within one instrument such as a Roland JD 800 there are 4 tones per key. You press a key and you can hear up to 4 layers at once. What do we put on those layers now?
Suppose we want to make a big fat expansive pad type of sound. You would start with maybe a warm fat pad type sound but you would not use the same sound on all 4 layers necessarily.
(or even 4 slightly different pad sounds) You could, and you could pan and detune them and filter and effect them differently and yes you would end up with a more expansive sound compared to say one pad sound on its own. But not a lot bigger though and you have wasted the other three layers to a certain degree.
But the real fun starts when you layer
4 quite different things (eg a pad sound plus a slow white noise effect or a reverse cymbal down 4 octaves plus bell like tone down one octave but with slow vibrato being added plus some tinkling wind chimes etc..) in order to make a very total expansive sound. And after that you apply very different filter and effects setting to each layer and balance them as well. Now the sound will 4 times as interesting as it was before! If you were using compressors on each layer then they too would need to be best set for the sound on that particular layer.
If I wanted to make a string sound with 4 layers then I would use all string sounds yes but all very different sounding string patches.
(from 4 different samplers or VST' is best) That yields the best result overall. It is a bit like the reason why getting one synth to play 16 parts does not sound nearly as strong as 16 different synths playing those same parts.
TIP* Why checking/mixing on a singe small mono speaker at lowish volume is so cool: If you have got say 4 or 5 guitars similarly layered
(even played differently in each case) When you mix and line up all these sounds behind each other so to speak in a small mono speaker at low volume the net effect is a small guitar sound and you cannot hear the individual layers very well. The small speaker cannot be fooled see.
But
record and
treat all those layers differently and in the small speaker in mono this composite sounds now gets very big very quickly even in mono! eg during tracking: Different guitars, pickup settings, effects and amps, cabs and mics and pres on the way in to your DAW and different processing on each layer after as well.
You don't have to change up too many variables either to get an effective result. Even of you only had one guitar with care you could make it sound quite different in each of 4 layers. I recently mastered an album where every instrument and track was recorded with only one microphone and that was a Rode NT1A. But he changed the guitar each time and the mic positioning and room and the resultant guitar tracks are completely different from each other despite using the same mic every time.