This stat just calculates the amount of minutes a player has played out of the maximum minutes that they could have played for their team. MP% is slightly better than just minutes per game because it can factor in overtime games. The calculation is simply Player MP / (Team MP / 5).
The usage rate for player estimates what percentage of their team's possessions were they responsible for ending, either with a shot, a turnover, or getting fouled. This stat is useful for determinig how involved a player is while they are on the floor. Getting minutes is certainly valuable, but if a player is not being used at the end of plays, they're probably not as important as a player that plays a lot and is actually used in the offense. The calculation I use for usage rate is: [(FGA + TOV + 0.475 * FTA) * TeamMP / 5] / [TeamMP * (TeamFGA + TeamTOV + 0.475 * TeamFTA)].