Why this is important to visit graves of Tzaddikim?

There is a custom to visit and pray by the graves of tsaddikim as the Talmud (Sotah 34b) teaches that Joshua prayed by the graves of our fathers in Chevron, to ask God to answer our prayers, if not for the sake of our own actions, at least for the sake of our righteous fore-fathers. In addition, any connection we have with our predecessors is important, for Jewish tradition is based upon this un-broken chain, which we have with previous generations This is not one of the 613 Torah commandments, and not even a rabbinical mitzvah, but rather a worthy custom, yet the main point of prayer is to have a relationship with God, and purify and improve ourselves.