The tactics and calculation training is the most important type of training a chess player can undertake if he or she is serious of his or her chess improvement.
The reason for this is that all chess games are decided by tactics. You cannot win if the opponent does not make a mistake – hence you have to take advantage of that mistake. While the mistake can be of various types, from the one-move blunders to a “small” positional detail 4 moves into a line, the mistake is always tactical in its essence. Something was missed and the opponent can take advantage of that miss.