Full Explanation of why we are now using user-agents: https://en.bitcoin.it/wiki/BIP_0014
TL;DR explanation: Previously, we have used the protocol version field to keep track of what version of the Bitcoin Client people were using. This is a simple integer. This is bad because all other Bitcoin implementations would have to stick to the Satoshi-client's version number even if the protocol didn't change. There also was previously no way of finding out what client people were using. BIP 14 solves this.