Why do you need an exit Plan?

As an entrepreneur, why do you want to have an exit plan? If your goal is to touch the lives of people around you and your idea is a tool to achieve that, why do you want to exit out of it.

On a different note, I follow Alok Kajrewal and his blog and saw this title – ‘Can I have an Exist please?’ Alok brings more information (read experience) on planning for an exit. It is an interesting read, but my question (and afterthought) prevails.