It is ironic how this guide is focused on outdoor projectors yet you still may not know why they are ideal for outdoor use. Well, to be honest, for the most part, it is because of their lumens as some outdoor projectors will be too bright for indoor use. But, overall, they are designed to be used outdoors. However, it still may be wise for you to protect them while they are outside and it goes beyond protecting them from storms. The real enemy of projectors is condensation and when it constantly turns on and turns off, the internal mechanical components will degrade from the exposure to moisture.
One surefire way to prevent this is to invest in a projector enclosure along with a climate control system. Yes, it sounds like a lot but it will be necessary if you plan on leaving your projector outdoors.