After you have finished writing your paper, reread it and summarize the main parts. An abstract is useful if your paper is going to be published in a journal, posted on the Web, placed in an abstracts data base, or put in any position where potential readers might make a decision to read your paper. It might be required as part of a Final Year paper. For shorter assignments submitted within a classroom or course context it would probably not be required. Check with your teacher.
Abstracts use predictable language.