It makes sense 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 was damaged during your recent trip. If you have a spare lamp, you can change it immediately. Problem solved!