The mitzva of guarding the matzot was stated about the matzat mitzva we are commanded to eat on the night of the fifteenth of Nisan. The idea is to honor the mitzva by guarding them especially carefully. In other words, matzot that were not guarded but about which there is no concern that they may have become ĥametz may be eaten on Pesaĥ. They are just not fit for use as matzot mitzva since they were not guarded specifically for that purpose.
Thus, throughout Pesaĥ, it is permissible to eat matza that is kosher for Pesaĥ, even if it is not shmura.
Nevertheless, some are meticulous about eating shmura matza throughout Pesaĥ. There are two reasons for this: one is that some poskim maintain that while there is no duty to eat matzot throughout Pesaĥ, one who does so fulfills a mitzva; therefore, one should eat shmura matza. According to this, it is sufficient to eat a kezayit of shmura matza at every meal. Similarly, it is sufficient to use our regular matzot (not labeled as shmura), which have been guarded from the time of grinding, because, as we have learned, one can fulfill the requirement for matzat mitzva with matza that has been guarded from the time of grinding.
The second reason for eating shmura matza is concern about ĥametz. Out of all the foods we eat on Pesaĥ, matza is the likeliest to become ĥametz. Therefore, if the wheat grains are not guarded from harvest time, there is concern that they may have become ĥametz. Thus, there is good reason to be stringent and to eat only matza that has been guarded from harvest time.
Today, the advantage of matzot that have been guarded from harvest time is not only in their having been protected from contact with water since the grain was harvested. In general, much more care is taken throughout their entire manufacturing process. For example, the baking machines are stopped every eighteen minutes for a thorough cleaning. Thus these matzot benefit from a whole series of enhancements.
In sum, regular matzot, which have been guarded from the time of the grinding, are kosher for all of Pesaĥ, le-khatĥila. Even those who maintain that there is a mitzva to eat matza throughout the seven days of Pesaĥ agree that one fulfills the mitzva with such matzot. Those who are more scrupulous eat matzot that have been guarded from harvest time, primarily because these matzot are more carefully guarded against becoming ĥametz.
. The custom of the Vilna Gaon was to eat shmura matza the entire Pesaĥ for two reasons. Firstly, he maintained that there is a mitzva to eat shmura matza all seven days, but in this regard he reasoned that it only needs to be supervised from the time of kneading. Secondly, he was concerned about ĥametz, and, accordingly, supervision would be necessary from the time of harvesting. This is quoted in BHL 453:4. AHS 453:20-23 states that according to Rif and Rambam, there is a rabbinic mitzva to guard the matza from the time of harvesting to prevent it from becoming ĥametz; accordingly, this applies to all matza that one eats over Pesaĥ. However, according to most poskim one need not eat matza that was supervised from the time of harvesting, and this is the implication of SA 453:4 and MB ad loc.
It thus emerges that the primary enhancement of the mitzva is to guard the wheat against becoming ĥametz from the time of harvesting. Thus, the kernels are harvested before they dry out to prevent any concerns of rainwater potentially making them ĥametz. The kernels are then stored in a dry place, preventing any kernels from puffing up or splitting, which would indicate that they have started to become ĥametz. On the other hand, wheat imported from abroad, from which all other matza is made, sometimes contains kernels that have already become ĥametz. Even though matza bakers are careful to purchase clean shipments, there is still a concern that some ĥametz kernels mixed in. And even though these kernels are batel be-shishim (because the matza is baked before Pesaĥ, the ĥametz is not ĥozer ve-ne’or on Pesaĥ according to most poskim; see above 7:3-4), this matza is as ideal as that which is supervised from the time of harvesting. Since matza that was guarded from the time of harvesting is the best and most special, bakers are careful to perform all of the other steps in the most meticulous way possible: during baking, they stop the machines every eighteen minutes and clean them thoroughly; they constantly declare that they are working le-shem matzat mitzva; and they are more careful to supervise the entire process. Conversely, the trend with regard to non-shmura matza is to produce and sell it as cheaply as possible, so that the general public will be able to afford it. Therefore, only the mixers used for kneading are changed every eighteen minutes, but the rollers are cleaned while they are running. Although they are cleaned in a way that ensures that every part is cleaned every eighteen minutes, since the cleaning is done while the machines are running, it is more difficult to clean the machines thoroughly.
Yet shmura matza is far more expensive than the regular matza (even if it is only guarded from the time of grinding the wheat, and can thus be used to fulfill the mitzva of eating matza according to those who maintain that there is a mitzva for the entire holiday). Some say that it is preferable to give charity to the poor or to support Torah study than to buy shmura matza. On the other hand, why shouldn’t one who spends lavishly on furniture, clothing, and other luxuries spend extra money to fulfill mitzvot in the best possible way? Ultimately, in practice, regular matza is kosher, and one may rely on the fact that the kashrut supervisors do their jobs properly. When it comes to fulfilling higher standards of mitzvot, one must decide if and how he wishes to go beyond the letter of the law.