A projector holds a delicate lens and, in many models, a fragile, expensive lamp that is especially vulnerable when hot. Moving it before it has cooled, or without protecting the lens, is how the optics get damaged. Letting it cool fully, capping the lens, and cushioning the body carefully keep a projector sharp and working after the move.
Pro tip: The original hard case, if you have it, is the safest container for the lens and lamp.
Watch out: Never move a projector while it is still hot. A warm lamp is fragile and can crack from the jolt.