Opera houses are designed to sound right when they’re packed with people. Singing to an empty opera house will sound wrong to the people listening to a stream or recording, and to the singers which might throw them off.

You could pack the room with some other material, but plants work!

While the idea itself might be consider 'gimmicky', almost every study that has been done on the matter shows that music positively affects plant growth.. which is not something that most people are aware of.

(this is believed to be due to the vibrations in the air mimicking the wind)