Answer: Just as a father would not expect his son to do anything without first giving him clear instructions, so too G-d would not expect us to do something without first telling us clearly how to do it. The fact that the bible is ambiguous and vague compels us to say that the bible was not given alone - it came with an explanation. In this week's parshah the Torah relates that although Moses went up the mountain to receive the Torah, it was not until forty days later that he descended. Moses stayed up on the mountain for forty days and forty nights where G-d dictated to Moses the written law ("The bible") and also orally described details of the 613 mitzvot and how to perform them. This is known as "the oral law".
Kabbalah explains this phenomenon in the most beautiful way: Just like in humankind there are men and women, so too the Torah contains both a "masculine" side and a "feminine" side: The written law is "masculine" - just like the father's contribution - the seed, the cell from which 613 limbs will sprout, the fundamental structure for Judaism. The Oral Law is "feminine" - like the mother's part - the extension of that seed into a healthy, developed and well understood Judaism.
When you read the bible alone you only get half the story. The Oral Law is made to complement the Written Law, together giving us a very clear picture.