One of the golden rules of presenting is that – at some time in your presenting career – something unexpected is going to happen. Someone will heckle you, the microphone will die, someone will wander across the stage. Whatever it is, your ability to react in a positive way will influence your credibility in the eyes of the audience.
Improvisation is an important skill for all presenters. Born out of the theatre, it is a great way to build creativity, listening and teamwork skills. For a presenter it allows you to smoothly move forward and manage anything unusual that is thrown at you (can help with Q&A sessions too).
Learn about Improv (short for improvisation) at Wiki.
See some great improv resources at Story Robot.