When the speaker has finished covering the body of the presentation, it is necessary to conclude. This is the opportunity to reiterate the main topic of the presentation.
It may seem unnecessary for the speaker to restate the topic, after all, has has been paying attention. However, the audience has now been sitting for a significant amount of time and have been introduced to a number of ideas. Additionally, no presentation is perfect so there will be distractions and each individual may have had their minds wander. In an audience with n people, there will be n different concepts as to what was most significant in a particular presentation as each person will interpret what is said in a slightly different way. The original outline has, by now, been forgotten (short-term memory only stores 6-10 concepts) so it is now necessary to once again emphasize what the the speaker believes to be the core topic of the presentation together with a summary of the justification:
"In conclusion, we have covered A, B, and C to justify why X is appropriate for Z."
where each variable is replaced by an appropriate word or phrase.
Once a presentation has been completed, it should be possible to complete a one paragraph abstract of the presentation. This should summarize the background as a single sentence followed by the objective or thesis statement of the presentation. This should be followed by a few sentences which summarize the body of the talk.
An abstract is useful both for the speaker—it ensures the speaker understands what he or she is attempting to convey—and a potential audience. Conferences often require speakers to present an abstract and audience members will often choose to attend or ignore a presentation based on the abstract.