It is always good to keep a spare lamp on-hand because you never know when you will need it until you turn on the projector. You might be preparing your quarterly presentation in the executive boardroom, warming up for a sales pitch for your most important customer, or sitting down to watch a film with your family. Suddenly the excitement of being prepared is interrupted by the realization that the lamp in your projector is just a little too old, or maybe it took too big of a bump during your travel. If you have a spare lamp, you can change it immediately.