Usually the backup in my experience is for clearing a lane on Boss Island with the best possible champs whereas lane clearers can't always take champs for their lane and ideal champs for bosses. Unless someone fails and needs help of course. Never heard of this being used in Silver, but at least they are trying to be organized.
I joined a Silver 1 alliance last week and I've been told to play as backup, other members were assigned specific paths. What does that even mean.