Phosphate is a mineral and compared to the other compounds of barley is the amount
of minerals pretty low. But the minerals are important for the metabolism of plants (very useful while malting) and for some brewing technical matters later on. Zinc, for instance, is responsible for a healthy and active yeast, oxalates for beerstone and especially the phosphates are involved with the pH-value in mash, wort and final beer. Minerals in wort are almost completely influenced by the malt and neglectable by the brewing liquor. Admitted, the brewing process itself has a certain influence to the mineral content in wort (eg amount of sparge liquor), but it still depends mainly on the malt. Phosphates have the property to accept or to donate, up to a certain point of course, H+-ions or other protons. That means, they can react both as an acid as well as a caustic. This function is also called buffer effect.
Every enzyme has its optimum pH and temperature. On the other hand the modification of the endosperm is influenced by the mash-pH as well. So the pH is an important factor witch does influence the mash and beer quality. Franke and Windish were the first people in the beginning of the 20th century which concerned with this fact. They wanted to compensate the disadvantageous characteristics of brewing liquor with a high residual alkalinity. The mash pH influences the lautering time, the leaching of the husks, the processes by wort-boiling-, the isomerisation of bitter substances, the formation of cold and hot trub and the pH decreasing during fermentation.