Block Outgoing Connections. Why? Necessary?

Every time you click on a spam e-mail to delete it, it loads images from the server which the server can then cross-reference and identify you a valid e-mail address. That is the primary reason I have it.

Every time you click on a spam e-mail to delete it, it loads images from the server

If the message is identified as junk then Mail.app doesn't load image, unless you specifically tell it to do so.

There are other reasons to block, though, with malware being the most invasive - if you have malware on your machine it's likely to initiate outgoing connections which Little Snitch will identify.