"The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime.  Because of that, I wanted to design it to support every possible transaction type I could think of.  The problem was, each thing required special support code and data fields whether it was used or not, and only covered one special case at a time.  It would have been an explosion of special cases.  The solution was script, which generalizes the problem so transacting parties can describe their transaction as a predicate that the node network evaluates.  The nodes only need to understand the transaction to the extent of evaluating whether the sender's conditions are met."

Satoshi Nakamoto
BitcoinTalk
17-06-2010

https://bitcointalk.org/index.php?topic=195.msg1611#msg1611