There was once an inventor who built what he claimed was "the perfect house." The building was hermetically sealed against outside elements. He carefully designed and painstakingly constructed the walls, roof, windows, and doors. There were no openings that allowed the wind to enter, no small cracks where dust and dirt could sneak in.
When the house was finally finished and the proud owner moved in, his new neighbors noticed that the inventor came with some brooms, dustpans, mops and other cleaning equipment.
The neighbors were surprised. "Why do you need brooms and mops? Didn't you say that no dust and dirt can penetrate into your wonderful new house?"
"It is true", the man replied, "that my new home is 100% airtight, without the smallest crack or hole. But that does not mean that I will never need to clean it!
"You see, there will be guests and visitors entering the house. Naturally, they will bring in dirt with their coats and shoes. If I didn't routinely clean the house, it would eventually become full of dirt."
The soul resides inside the body, like a person in his home. And like a well-designed house secure against outside elements, God gave us various mitzvot to watch over those organs and limbs that have contact with the outer world.
We are commanded to be careful what kind of food we put in our mouths, what we let our our eyes see, what kind of speech we let our ears hear, and so on. By conscientiously observing these mitzvot, we prevent dirt and impurities from the outside world from entering the body's "windows" and contaminating the soul's natural purity.
Nonetheless, even those who observe these mitzvot carefully still need to make a living. We all eat and socialize with others. Because of this contact with society, it is impossible not to be somewhat sullied by the impurities of this world.
Therefore, like the inventor of the airtight house who needed brooms and mops to periodically clean his home, the Torah gave us one day in the year - Yom Kippur - to purify our hearts and cleanse our spirits. All of us need this day of atonement and purification, even the holiest and most saintly.
(Adapted from Mishlei Yaakov, pp. 233-234)