In the simplest sense, a separation of duties. If the judge was to be the sole decider of what is worth consideration then you have this individual functioning as an unquestioned king.

No single person is going to know every case, consider every angle, and have an inclination to pursue all points of view. Even the supreme court receives amicus briefs to guide a case.

An attorney has a duty to get the best possible outcome for their client, even if they disagree with what the client may have done. A single person judiciary could never have that position because they are effectively advocating both sides to themselves.