While you can make “great” beer without worrying about pH, brewing truly great all grain beer relies on understanding the concept of mash pH and its impact on beer.
Mash pH is very difficult to predict in advance. Though some large scale commercial brewers are able to accurately predict mash pH for certain recipes, they are often brewing the same recipe with exactly the same malts under exactly the same conditions repeatedly. Unfortunately variations in malts and water make this impractical at the home level, where often we brew with different recipes and ingredients every single time.
Some sources of the factors that affect pH include:
Water Used: The chemical composition of the water is a large driver in mash pH. Calcium, magnesium, carbonates and bicarbonates are all drivers of mash pH. While I can’t cover a complete course in brewing water or residual water alkalinity here, its important that you have good brewing water to brew with. Most water sources are slightly alkaline, so they tend to drive the mash pH above the desired 5.2-5.4 range.
Malts: Malts tend to be acidic, which means that they lower the mash pH of the overall mixture. Dark malts in particular can be very acidic, which is why many darker beers require little mash pH adjustment. Lighter malts have less buffering capability, so lighter beers often need additives covered blow to drive the mash pH down to the desired level. Unfortunately the acidity of malts varies widely and is not measured and published, so prediction of mash pH in advance is very difficult.