According to the late Rabbi Aryeh Kaplan, in ancient times – as late as the classical Greek period – many garments were four-cornered. Rather than tailored, clothing generally consisted of a simple rectangle of cloth, straight from the loom and worn as a shawl, cape, tunic or toga. Similar garments were worn in Talmudic times. Since everyone wore four-cornered clothing, they fulfilled the commandment just by tying tzitzit onto their everyday garment.
Because we no longer wear four-cornered garments, a special garment, the tallit katan, is now worn in order to fulfill this important mitzvah. Rabbi Yitzchak Abarbanel (Lisbon 1437-Venice 1508), wrote that this is why the Torah states that we must “make tzitzit… for all generations.” Though a time would come when four-cornered garments are no longer worn, we must continue to wear a special garment, i.e. a tallit katan, in order to fulfill the commandment of tzitzit.
Is it mandatory to buy a tallit katan?
The Rambam notes that it is not mandatory to buy a tallit katan in order to wear tzitzit, but a punctilious person should not seek to exempt himself (Hil. Tzitzit, 3, 11). The Rambam then writes, “One should always carefully observe the mitzvah of tzitzit, for the Torah says it encompasses all of the other mitzvahs, as is written, ‘And you shall see [the tzitzit]…'”
According to the Bach, “Although the verse does not seem to indicate one must buy a tallit [katan], nevertheless it appears that it is a big mitzvah to buy and recall the mitzvahs upon seeing [the tzitzit].”
In a famous responsum Rabbi Moshe Feinstein wrote to his son Rabbi Dovid Feinstein, shlita, the elder Rabbi Feinstein agrees with his son’s analysis that although there is no absolute halachic requirement to wear a tallit katan, there is a binding custom to do so.
The Chayei Adam writes that wearing tzitzit is so important, one shouldn’t be without a tallit katan for even a moment of the day, rather he should don a tallit katan as soon as he wakes up, and not even walk four cubits without tzitzit. “And at the very least one should wear tzitzit during the [morning prayers].”