Why you need to control light in a projector based Home Cinema

I was doing an install up in Hua Hin, Thailand recently and had to do most of the cinema setup during the day, it's a dedicated home theatre room so it was easy to keep the curtains closed.  At one point during the install whilst I was tweaking the on screen image someone opened the curtains momentarily.

It served as a reminder as to why you need to control light, I decided to take the following photos of the screen showing exactly the same image, 1st with the curtains closed and 2nd one with a curtain open. The screen completely washed out, I couldn't discern any image whatsoever.  This is a bright-ish projector on a Gray screen.

Projection Home Cinema with curtains closed

Projection Home Cinema with curtains open