Milling can be a successful strategy due to the fact that a player who can not draw a card from his library when he is required to, due to the library not containing any cards anymore, loses the game. In addition, milling cards usually renders the milled cards unusable giving the defending player less options. Since the graveyard is also a public zone, it makes it also easier to calculate what options the defending player still has available.
However, due to the difficulty of milling enough cards and the circumstance that milling neither provides direct card advantage nor influences the board situations, milling is rarely seen as a tournament strategy outside the realm of combo decks, e.g. decks that win by Brain Freeze.
Regardless, milling your own deck has become a viable tournament strategy using the mechanic Dredge and the card Life from the Loam in particular. Hermit Druid and Cephalid Illusionist (in Combination with Nomads en-Kor) have also been used for this purpose. This efficiently makes it possible to get cards into the graveyard and utilize them from there e.g. Reanimation targets, Ichorid or simply lands from Life from the Loam to regain with that card and discard them for cards like Seismic Assault or Raven's Crime. It also provides the ability to grow Lhurgoyfs such as Tarmogoyf or to feed Psychatog.