Why does the Observer appear in every Fringe episode?

In one of the season 2 episodes Olivia asks Walter who they are and what they do and at the end of the episode you find out that their main role is to keep balance between the two universes. Hence why they were tailing Walter as when took peter from the alternate universe I'm order to try and save his life he disturbed the peace and now the observers were making sure he kept his side of the deal and let peter go when the time came as that was how to restore the balance.