pebbles or gravel at bottom of substrate

Some of the substrates described in TAG volumes 4 & 5 (that's
as far as I've read yet) had pebbles, stones, sand or gravel
in a layer beneath soil. I was wondering if anybody could supply
the thinking behind this? In natural environments there are
often several layers of substrata the lowest of which is 
similar to rocks & gravel; maybe they sought to emulate this?

Another thought I had was that one might want to provide
layers in the substrate where different sorts of redox reactions
would occur; I am thinking of Fe & Mn reduction to a soluble
form and nitrate reduction to the more usable form of ammonia.
The lowest redox level is where there is less oxygen or chemicals
which act like oxygen in reactions (to over-simplify). It
might be better for these minerals to be reduced in a layer
without the presence of a lot of plant roots. 

I was also curious if anyone had done redox potential measurements
in aquaria and if very low redox levels even occurred except
under extraordinary conditions?

Steve in Vancouver BC where the heat wave is over for now.