The nature of crust, the upper crust and what have you.

I was told when I was young eat up your crusts and I think the 'or else' bit was 'you won't get curly hair. I'm unsure of the imperative but definitely crusts were left when we were kids. Now I'm inclined to wonder if some in the innocence of childhood there was a wisdom to all that.
Crusts are something of an adult thing with boutique loaves pouting proud on bread stalls in shops and markets everywhere and demarcate themselves from the pallid regularity of mass produced and sliced and wrapped versions.
A crust is de novo a thinnish and dryish condensation of the softer  mix it is covering. In the presence of modest cooking or heat it remains thin, supple and pretty much the same colour with a little accentuation of the taste of the mix. With more and more heat it darkens considerably through to black, thickens and stiffens so much that it becomes brittle and sharp edged. The taste sharpens from an enjoyable  caramelised taint to the outright bitterness of carbonised dough.
All this is just taste you might say but of course this is true as long as you take on board the rider that 'burnt is bad'. thus:
   More information about this is in a very helpful summary by the USA National Cancer Institute. Note that one of the studies quoted looked at cooking at 200ºC or 250ºC, which is much hotter than ordinary baking/roasting. However, that is the kind of temperature you use to cook a pizza... Apologies to the reference in that it is me who has emphasised pizza, for obvious reasons of course.  
It reasonably follows that heat producing our bread and gently cindering the outer to us the so much liked crust is doing the same to that food.