#pH# is very important in biological systems, including ours as a change of #pH# will easily kill of microorganisms or stop enzymes from working, so your body will try its best to keep its parts at their appropriate #pH#s, the stomach is acid, the others are neutral-ish, and so on.
A bunch of common foodstuffs are either acid or basic, for example, all citric fruits and their juices are acids; vitamins C and B9 are acids, vinegar is an acid, which means that #pH# is a simpler way to check the quality or quantity of these items.
Anything with water has a #pH#, including things like the sea, pools and drinking water, so checking and mantaining a certain #pH# range in those things are important to make sure those drinking or bathing in these waters won't get sick.
But in the end, asking what is the importance of #pH# is the same thing as asking what's the importance of trig or the importance of #F = ma#, it's hard to explain exactly because the average person will likely not have much contact with it in their lives except the vague notion that it exists but it's something very important behind the scenes.
