Following my previous question: how is it possible for one substance to act both polar and non-polar? how is the polar/non-polar nature of a substance useful?

when the substance contain the two parts (polar and non-polar) like soap.

Explanation:

Soap is produced from the basic hydrolysis of ester "saponification", it is considered as a salt of hydrocarbons. The soap molecule contains the two parts: a polar group  (-COO-Na+) and a non-polar group (R-hydrocarbon part). the polar group is called the head and the tail is the non-polar group. The polar group is hydrophilic in nature (water loving) and the non-polar group is hydrophobic in nature (water repelling).The polar/non-polar nature of a substance is useful that it can dissolve the non-polar substances by the polar part and also can dissolve the polar substances by the non-polar part. It commonly used in detergents and soaps.