Learn a little about data security.
SSL is a means of keeping your information safe while it's being transferred through cyberspace. SSL stands for Secure Sockets Layer, but if we go into those details it's a reasonably safe bet that your eyes will glaze over by the time the first sentence finishes. The crux of it is that your computer encrypts your message before it gets sent. Anyone who intercepts the message will find it useless without the key to decrypt it, which is transferred separately.
When data is transferred unencrypted, anyone along the way who intercepts the data such as a hacker or nosey neighbour with a bit of nouse (know-how) can read it. However by first establishing a secure connection to the server then transferring the message in an encrypted state, anyone who intercepts it will not be able to read the information within. Your bank uses the same method when you do your online banking.
Using SSL keeps your personal information safe from prying eyes who might be looking out for anything from credit card details to trade secrets, or even the very username and password you use to get your emails (leaving your account open to use by spammers or identity theft). It is extremely recommended that all emails be encrypted.