pH does *not* have just one isolated function in the mash nor does it play one major role, instead it has many signifcant roles.
There is a list of how the pH affects the mash here:
http://braukaiser.com/wiki/index.php?title=How_pH_affects_brewing
Not all are applicable to the mash but the mash is where it begins:
- The enzymatic activity in the mash is increased as all important enzymes get activated. (except for alpha amylase which starts to suffer at a pH below 5.6)
- More zinc, an essential yeast nutrient, goes into solution
- The extract yield (efficiency) is improved
- The protein coagulation and precipitation is improved (improved break formation)
- The redox potential is improved which results in a lower susceptibility to oxygen.
- The run-off speed is improved
- The color increase during the wort boil is reduced
- Better trub precipitation and faster pH drop lead to faster fermentation and greater attenuation of the beer.
- Lover viscosity improves filterability
- The taste of the beer is more rounded, fuller and softer. The beer is crisper, more fresh and shows more character.
- The hop bitterness is more pleasant and doesn't linger
- The foam is more stable and denser
- The color of the beer is lighter
- Mash oxidation is reduced since the main culprit, the lipoxigenase enzyme, doesn't work well at low mash pH conditions
- Haze stability is improved
- Beer digestion is stimulated. This is a positive effect of the lactic acid
- Susceptibility to microbial spoilage is reduced through Lower beer pH: beer spoilage organism don't grow below a pH of 4.4
- Higher attenuation
If you mean what does adding grain to a neutral water do, then all grains acidify and drop the pH. There are various benefits at different pH values.