I'd expect that most coaches of existing teams know who they want to keep and what the weak areas are for a team (i.e. gaps to fill). So, especially as kids get older, the tryouts are probably more about filling gaps.
Also remember that the coaches are trying to put together the best team. So that means the best 11 players for a eam may not always correspond to the best eleven athletes etc. All positions must be filled with the best players for a position. For example, if the "best eleven players" were all keepers and midfielders, then likely the coach would have to look for defenders and forwards in lieu of a few of the worst of the top 11.
