It's not always possible to interpret the songs as easily as one might expect. It might not even be easy to say the words. You have to listen. Knowing enough to be able to start on song on the right pitch, to say nothing of remembering how the tune goes or the words, takes a great deal of time and dedication. That's where leadership comes in.
Leadership is the key to the music.
If all twenty singers were sitting with you right now, probably three could start the songs. The rest could just follow. Even though they know the song once it starts, they don't know how to start it. Normally a leader is the first person asked when a performance is to be held. The singers sing for a variety of reasons.