In this week’s portion we have the mitzva of tzitzis — the obligation to attach strings to any four cornered garment: "G-d spoke to Moshe saying: Speak to the Children of Israel and say to them that they shall make themselves tzitzis on the corners of their garments throughout their generations. And they shall place upon the tzitzis of each corner a thread of techeles (a blue like color) wool. It shall constitute tzitzis for you that you may see it and remember all the commandments of G-d and perform them" (Numbers 15:37-39).
As we know, each and every commandment has a distinct purpose to it. What is the purpose of tzitzis? Tzitzis is an opportunistic mitzva. Only when one is wearing a four cornered garment is there an obligation to wear tzitzis (today, as most articles of clothing do not have four corners, we wear a special four cornered garment so we are able to wear tzitzis). The mitzva of tzitzis is obviously connected to the idea of clothing.
What do clothing represent? The first time clothing were introduced was as a result of the sin of Adom and Chavah (Adam and Eve). The Torah relates to us that after they ate the forbidden fruit, they noticed they were not wearing anything and felt ashamed. Why did they not feel ashamed previous to their committing the sin? By eating from the forbidden tree, they invited the evil inclination into their body (as opposed to before when the evil inclination was external, as manifested by the snake). By doing so their body changed from "clothing" for the neshama (soul), i.e., the positive inclination, to a potential vehicle for the evil inclination. And when this happened, they felt ashamed, for they saw the potential for evil in their bodies. In fact, the Hebrew word for clothing is "Beged". Beged also means betrayal. Clothing were only necessary after their betrayal of G-d (heard from Rabbi Yaakov Pollack).
So the role of clothing is to cover up. When the evil inclination came into the body, it also introduced to man the concept of judging something from a purely external level. In order to prevent that from happening, we must "clothe the clothing". The two parts of the body that clothing is not meant to cover are the hands and the face. These are the two parts of the body which are best potentially used for spiritual endeavors, and which are totally different on each person.
Tzitzis are a reminder placed on the clothing. They remind us why we cover up. They remind us of the commandments that sanctify the body, to use the body for physical purposes that enhance it spiritually.