The Gemara states that one must recite the blessing for a mitzvah prior to performing the mitzvah. This is referred to as over laasiyasan. There are a number of instances in the Torah where the word over is used and the Medrash and Gemara interpret the word to denote anger. An example of this is regarding the laws of inheritance, where it is said, vhaavartem es nachalaso lebito, and you shall cause his inheritance to pass over to his daughter. The Gemara derives from the usage of the word vhaavartem that HaShem is angered by one who does not leave over a male child to inherit his estate. The same interpretation can be applied here. One must recite a blessing over laasiyasan, i.e. to prevent anger. We learned earlier on 38a that the remnants of a mitzvah can prevent punishment from occurring. It would follow then that although we rule that blessings do not prevent the fulfillment of the mitzvah, one should certainly be meticulous in reciting a blessing prior to performing a mitzvah, as the recital of the blessing will afford him protection and ward off any harmful agents.