"Early science, particularly geometry and astronomy/astrology, was connected to the divine for most medieval scholars. The compass in this 13th century manuscript is a symbol of God's act of Creation, as many believed that there was something intrinsically "divine" or "perfect" that could be found in circles."(Reference: Wikipedia for circle)
The Kara is a gift from the guru to constantly remind us that our will at all times must be god-like. the Kara in every aspect represents God's true nature. we wear it on our hand. we, to politely put, 'wear god' on our hand so that every action we perform is one fueled by pure thoughts. yes, god is everywhere, and yes, god is in each and every one of us. but how often do we look within?
In the tenth month, you were made into a human being, O my merchant friend, and you were given your allotted time to perform good deeds." (Sri Guru Granth Sahib Ji page 76)
Every man is born to make another's life. no matter how unimportant we feel our lives are, we affect the people around us. a kind word. a gesture of good faith. the stand for all that is right. that's all. if only with every breath we took, we remembered that.
This is the true nature of the Kara. this is why it is one of the 5. now can we all stop referring to it as a 'ward for evil spirits'?