I just read this again and besides the general data put in other words I don´t think I answer your question
. I´m going to try again:
There is nitrougenous components in your wort this are: protein, polypeptides, aminoacids and nucleotides. Of this group, aminoacids are key to fermentation perfomance and beer quality: during the early stages of fermentation the yeast will take up aminoacids and use them to grow and store them in vacuoles to be use later in their metabolic activity, this is mostly done at early stages when there is very little ethanol presence (ethanol inhibits aminoacid transport), also doing this in early stages yeast wins in a "competitive race" with others microorganism taking nutrients from the wort to deprive other possible competitors of them. It was a common believe that during mashing the protelysis (the breakdown of proteins into polypeptydes and aminoacids) stopped at 60 Cº (140 Fº) this is not true, the fact it´s that this breakdown of proteins into aminoacids it´s faster at the regular protein rest temperatures cited above but it is conducted anyways at higher temperatures.
Polypetides are responsable for head formation and depending how hydrophobic they are head retantion (hydrophobe is the proterty of a molecule for which it´s repelead from a mass of water).
So a protein rest will reduce the presence of complex and (i´msorry but i don´t know the term in english) "pre-proteins" into polypetides and aminoacids. The longer chains of proteins cause among other things chill haze. That´s why a protein rest helps those three things: clarity, head formation and retation and yeast nutrients. Not knowing your malt and doing a proccess that is not correct WILL lead you to thin watery beers with no head, the proteins will break down in aminoacids and not polypetides so no head or body there and a very high fermentable wort.
Depending if you protein rest at the low side of those temps or the higher side you will get more aminoacids or more polypetides. You should aim for a balance.
This been said there is not actual an imperious need of a protein rest with modern well modified malts, first because they are already modified and proteolysis will still perform at saccarification temperatures but your beer body, mouthfeel, claritity and the presence of nutrients for yeast will be improved with a protein rest at the correct temperature.
So to try to keep it short and simple: it will help you with both worlds nutrients for yeast and clarity and head only if it is done at the correct temperature for the correct amount of time depending on your malts, technique, equipment (there is so many variables). There is a lot more in your wort that carbohydrates (read: starches, simple and complex sugar etc) and with a single infusion mash you are only tweaking a very small part of your wort chemestry.
So that´s why I step mash and protein rest. I don´t know if this is the kind of answer that you are looking for or if my english makes any sense at all or if some of the terms are not well translated. I even doubt if I´m correct or not but my experience (not a very long one I only been brewing all grain for 4 months and only 14 batches so far) tells me that this is correct.
If you need more info about this I will be more than happy to adress you to some books.
Brew on!