In the beginning of your presentation, the audience is trying to figure you out, and is forming a broad framework about what it is you are talking about. Watch out not to go off on a tangent too early in your story, your audience is not ready for it. Later in your talk, once the overall framework is established, it is perfectly fine to go on a little deviation.
For those interested: in geometry, the tangent is a line or surface that just touches a curve. After the connecting point both lines separate (Wikipedia).