Once you reach a critical size then you are bound by AOC rules to at least have a Maintenance Manager in the role of a CAM. For the smaller operations/owners this independant role will ensure that if your maintenance company does not work in conjunction with an AOC and therefore does not hold part M approvals and you yourself as a small flying school or private owner do not hold said part M approvals then you will still remain within the required "Controlled Environment" but you will also have an independant eye on what is going on day to day with the maintenance from a practical point of view and a review of your DOC's with the practical experience of when to go ahead with in depth maintenance and when to keep it airworthy.