People have to understand that football is irrelevant. People think that when a team plays a formation, the formation will always be the same. Well, the truth is it changes. One simple example is the so popular 4-4-2. When the team is in offense the formation changes to 4-2-4 or 2-4-4, etcet. Another example is the 4-3-3 on defense it becomes 4-5-1. Formations are just used to organize the players on the pitch. It is not something that holds the players in their position till the game is over. Understand that the pieces in a formation move they don't stay in the same spot.

I am not sure I am following you. Who plays 2-4-4 or 4-4-2. The fullbacksa may join an attack momentarily, but this doesn't make it a 2-4-4. Their main duty is still to defend.

It's a valid point. Last season, Manchester United's attack was very fluid with Rooney, Ronaldo, Giggs, Tevez swapping positions regularly. Setting up a formation is more for players to realise which areas should be covered when somebody decides to go on a run so the general shape can be maintained. That way you don't get caught out by suddenly not having a player on the wing for a pass.