In order to understand the purpose of phonological rules, you need to understand what a phoneme is. Dictionary.com defines a phoneme as the basic sounds of a spoken language, which can also be responsible for understanding the meaning of words. For example, the "ch" sound in the word "chips" is a phoneme, as is the standard "s" that distinguishes singular from plural nouns, as in "cats" verses "cat." Phonological rules, then, are the rules -- whether written or unwritten -- that control how sounds change during vocal communication.
Not all phonological rules are codified, which means not all of them have been written down as official rules of a language. According to an educational reference available at PBS.org, the accents and dialects of a language -- which include tiny variations in pronunciation -- help to establish group identity. When you meet someone who speaks the same way you do, you have an instant bond, while differences in speech signal the need to be sensitive to possible cultural or regional differences.
Another purpose of phonological rules is to enhance language comprehension, both in everyday life and when teaching languages. As previously mentioned, a slight change in sound can change a singular noun to a plural noun, while a certain tone of voice can indicate sarcasm -- that the speaker means the opposite of what she says. Understanding this is vital to understanding what someone says when she speaks to you. Also, an understanding of phonetics -- which includes knowledge of phonological rules -- is a highly effective method of teaching English as a second language.
A final purpose of phonological rules is to define the normal patterns of human speech. When a person uses the phonological rules of his society in the accepted manner, he can communicate properly and thus learn and grow. For this reason, lack of developmentally appropriate phonological awareness, especially in preschoolers, can indicate a speech disorder. Knowing this can allow parents, preschool teachers, and pediatricians to spot developmental problems early, when they can be most easily dealt with.