The purpose of writing an abstract is to allow readers to quickly and accurately understand your work and to decide if your work is relevant to the purpose and topics of the conference. If your abstract is accepted, it will also help participants at the conference decide if they want to see your talk or poster.
Prior to the conference, all submitted abstracts are evaluated by reviewers or judges whose job it is to decide who gets to present at the conference. If your abstract is accepted, it gets printed in the conference programme, so that all attendees can find it, read it, and decide if they want to come hear or see your presentation (at a conference there will be many presentations happening at the same time). A well-written abstract will get you to present at the conference AND will also attract others to your presentation!
When you are submitting an abstract to a conference, it is because you would like to present your work or research to the people attending the conference. You would like others to hear about what you have been working on or what you have learned. So, the whole reason you are writing the abstract is to have a "short and sweet" version of your project presentation.
The judges who decide who will present a verbal presentation or poster at the conference don't have time to read through entire papers or long presentations for each submission - they ask for an abstract instead! Make your abstract so interesting and accurate that they say "We want this person to present!"