So hiding your actual ELO Style ranking was a move to prevent smurfing as well as direct comparison between people in different divisions. The ultimate goal of a division is to prevent you from being rank #1,000,000 (my current MW2 Rank). However, if your ELO rank was displayed it would undermine this goal.
Because ranking within a division is still important, Blizzard also introduced a points system so that people could be ranked with in a single division. This number is visible so that people can use it for personal goals (and to know why so and so is above them despite a win/loss number).
Finally, your Hidden rank is actually an estimate which is used to determine who your opponents are and what league to place you in. As points are only relevant to a given league, it is necessary to have such a rank to allow for placement during league advancement.