I believe that definitions are artificial distinctions and useful fictions that we create in order to communicate our separate realities to one another. Every language has words for which there are agreed upon definitions. Children learn general definitions first through listening, conversation, and, interaction with caregivers. Once children attend school they learn more definitive, structured definitions. Having generally agreed upon definitions for words amongst a group of people allows them to effectively communicate their ideas amongst one another. If you, the reader, did not understand the general definition of the words I am using now, or didn't understand enough English to utilize context clues to understand certain words then my argument would be completely lost on you.
Slang words are a great example of why definitions are artificial distinctions. In every language, and even from region to region that shares the same language, there are "slang" words or terms. The meaning of the word or expression has been slightly altered from the original definition, or a new word has been created completely. People within that region understand the new meaning and use it appropriately in communication with each other. People who are exposed to the term or interact with others who use it will come to understand the definition, how the word or term is used.
In conclusion, definitions are both useful fictions and artificial distinctions. Definitions are needed and provide a solid foundation on which to play with words, to share ideas, and challenge our minds. We learn the useful fictions so that we may understand that they are artificial distinctions.

Resource:
Porter, B. (2002). The Voice of Reason: Fundamentals of Critical Thinking. New York, NY: Oxford University Press, Inc.