What is the reason for the rule in requiring that all the elements comprising the offense be incorporated in the information or complaint?

There is a presumption that the accused has no independent knowledge of the facts that constitute the offense. It is needed to reasonably indicate the exact offense which the accused is alleged to have committed to enable him to intelligently prepare for his defense; and in case of conviction or acquittal, he may plead the same in a subsequent prosecution for the same offense.

Taken from People vs Tano (May 5, 2000) and People vs Barrientos (285 SCRA 221)