In fact, it's so intriguing that I looked up exactly what a stand-in's job is. From Wikipedia:

A stand-in for film and television is a person who substitutes for the actor before filming, for technical purposes such as lighting.

Stand-ins are helpful in the initial processes of production. Lighting setup can be a slow and tedious process; during this time the actor will often be somewhere else. Stand-ins allow the director of photography to light the set, the camera department to light and focus scenes. The director will often ask stand-ins to deliver the scene dialogue ("lines") and walk through ("blocking") the scenes to be filmed. In this way, a good stand-in can help speed up the day's production and is a necessary and valuable cast member on a film...

Stand-ins do not necessarily look like the actor, but they must have the same skin tone, hair colour, height and build as the actor so that the lighting in a scene will be set up correctly. For example, if the lighting is set up with a stand-in shorter than an actor, the actor might end up having his or her head in relative darkness.

So why would they need a stand-in for Matt if he's just going to be one of the crowd of 15 people who aren't in the Final 3? Why do they need to make the lighting appropriate for him?