[MatrixDweller]

In a home theater they are probably there for aesthetic reasons first and practical reasons a far second. From a design perspective they would tend to give a feel of an old public theater or hall.

Without using advanced steel or re-enforced concrete techniques a building that has a high ceiling, and therefore high walls, needs extra support so that the walls don't collapse. You'll see classic buttresses on the outside of churches and castles. They are there to make the wall strong and to make it not fall down.

Modern design may use steel I-beams to re-enforce the wall and roof structures. The higher the wall and larger the roof the bigger the i-beam is needed. The i-beam could extend inside the building rather than outside making the outside square. The inside walls would then need to wrap around the i-beams which could make it look like a "buttress".

A fancy theater or concert hall would want to conceal or decorate the protrusions to make things look better. Throw a sconce or speaker on it, drape it in curtains or make it look like a roman column and it suddenly has a more appealing look and purpose.