Rails "includes" Method and Avoiding N+1 Query

I don't understand the Rails includes method as well as I'd like, and I ran into an issue that I'm hoping to clarify. I have a Board model that has_many :members and has_many :lists (and a list has_many :cards). In the following boards controller, the show method looks as follows:

def show
  @board = Board.includes(:members, lists: :cards).find(params[:id])
  ...
end

Why is the includes method needed here? Why couldn't we just use @board = Board.find(params[:id]) and then access the members and lists via @board.members and @board.lists? I guess I'm not really seeing why we would need to prefetch. It'd be awesome if someone could detail why this is more effective in terms of SQL queries. Thanks!