First, take a moment to think about your bank. Aside from keeping your money in a safe, the bank maintains a system of record to track what you receive and what you spend. The bank tracks this for each of its clients, and though we cannot see the record, we trust that the bank is diligently and honestly tracking these transactions... But... what if you DON'T trust the banks?
One of the primary goals of Bitcoin is to create a system that doesn't require the trust of a central authority. To accomplish this, we need BOTH a system of record (the Block Chain), AND a way to ensure it remains visible to everyone. To satisfy the second requirement, Bitcoin's creator needed to get creative. If we were to keep the Block Chain only in the hands of a select few, we could never guarantee that they would keep it honest and transparent. In fact, the wider the distribution, the less trust is required. And if EVERY user possesses a copy, the trust required shrinks to zero.