Many people will agree that it is good to have an agenda or table of contents in a presentation. Reasons can include laying out clearly by telling your audience what topics are being covered and knowing what you are going to unfold next, length of the presentation and acting as a guidance for your audience.
But are they really necessary? Probably not in all situations. Here's a couple of thoughts:
1. True that without an agenda, audience may sit through a presentation lost in a maze and not knowing when it will end. However, it is also true that in most presentations, a time period or limit is always given or known by the audience beforehand. An agenda in this case, is not a solution for presenters who knowingly exceed the time limit. Instead, one should be well prepared by rehearsing your presentation thoroughly. If you have only 10 minutes to present a case, audience will start wondering when you are going to end if you exceed the time limit. And if you are only given a short amount of time for your presentation, an agenda may not be something you want to include on a slide.
2. A good presentation is akin to storytelling or movie. You start the presentation strong, capture the attention of your audience and bring them from one chapter to the next, gradually unveiling the mystery. In some cases, agenda may act as an intermission between an introduction and the rest of your slides, much like a commercial break, which may not flow well with your story, or a spoiler that spells out everything clearly and spoils the surprise.
3. When presenting for instance, a business idea to your superior or client, a handout of your slides may be required to be submitted to them before your presentation. In this case, an agenda is not necessary since they would have skimmed through your handout and briefly know the topics you are going to cover.
4. With agenda, audience can follow your presentation like a navigation guide, yet being a good presenter, you can become a driver and lead the passengers to the destination smoothly. The passengers do not necessarily need to know how to get to the destination, they just need to get there comfortably. Likewise, a bumpy ride will not give a good experience even if informed of the path that will be taken.
5. Instead of an agenda, one can use a progress bar similar to a website's shopping cart checkout process (example 1, example 2) which lists out the main topics and highlights the current ones you are presenting. By doing so, audience can know what you are going to cover without hurting the flow or time of the presentation.