Why the ratings used for different time controls are different? The FIDE assigns a player not one but three different ratings:


For classical games featuring long time controls, that is, 90 minutes for the first 40 moves + 30 minutes for the rest of the game + 30 seconds increment from move one, the players have a standard rating.
For games played with a time control of 15 minutes + 10 seconds increment, the players have a rapid rating.
For games played with a time control of 3 minutes + 2 seconds increment, the players have a blitz rating.


A similar method for rating players is also implemented by many websites like Chess.com and the ICC. Why is this done? Wouldn't it be easier and more efficient to measure the global strength of a player to have only one rating?

This question is based on one that was being closed because of being unclear what it was asking.
Different (though related) skills are involved in standard chess and blitz chess.

The standard rating is the one that really "counts" as far as people's lists of the best chess players goes. So you don't want to contaminate that rating, used for hours-long serious games, with results from 5-minute games. The blitz and rapid ratings were introduced recently so that players' games at those time controls can count for something without affecting their "real" standard rating.