I've seen posts where extract brewers didn't need to be concerned about pH. However, my well water pH is around 7.6, and is high in bicarbonate. I started to notice, I'll call it a "sour" taste, which I later identified as being astringent (like sucking a tea bag). I discovered that the high pH was extracting tannins during the grain steeping and sparging processes. It wasn't high temperature, which I know also causes this. Anyway, I started to adjust my pH by adding 1 ml of lactic acid to 2 liters of steeping and sparging water to get the pH down to around 6.
I've been doing this for about 9 months and my beers have tasted better. The "sour" astringent taste is gone. It was really strong on lighter beers like Kolsch and Pilsners.
Anyway, just relating my experiences as an extract brewer.