At the end of Parshas Shlach, we are given the mitzvah of tzitzis. Tzitzis are meant to remind us to do all the 613 mitzvos.
Despite this unusual facet, it is not strictly an obligatory mitzvah. Only if one has a garment with four corners is he obligated to add tzitzis strings to it. A coat with two corners and a rounded back, for example, does not need tzitzis.
However, even though it is not required for a person to purchase a garment that will require tzitzis in order to fulfill the mitzvah, this is indeed the custom, and one should try not to veer from this practice.
The Gemara (Menachos 41a) speaks of an angel which accused R’ Katina of trying to get out of wearing tzitzis and told him that though it isn’t a strict obligation, at a time of Heavenly anger, there would be punishment for one who did not wear them.
It seems that when Hashem’s anger is aroused, one who actively seeks to be obligated in mitzvos, and do more than the basic requirements of Hashem, will gain Divine protection, hence the custom to buy and wear a garment which needs tzitzis.
In Parshas Shlach, we angered Hashem numerous times, including the spies and the Jews trying to go up to Canaan without Hashem’s approval. Perhaps that’s why this mitzvah was given after these events, to afford us the protection we desperately need at a time of wrath.
