One of chemistry’s best tricks is to determine the concentration of various substances. Imagine buying vinegar and sometimes the acetic acid concentration is so weak that it tastes like water, and other times it is so strong that it burns the skin off the tongue and throat. So finding concentration is an important skill.
Many substances that we use are either acidic or basic. Because acids and bases can neutralize each other, that gives us a strategy for finding the unknown concentration of an acid or a base when we know the concentration of the other one.