Evaluation, calculation, decision making!

During the game, chess players constantly have to calculate variations.  Why is this done?  Oddly enough, the answer to this question in the chess literature was given only relatively recently by Grandmaster Boris Gelfand.  In his book "Dynamic Decision Making in Chess".  He writes "in the end we are calculating options in order to understand the position to the extent that is needed to make a decision, and not in order to recount the computer or flash at a press conference."