Sending one file is easier than sending ten.
Also, if you are zipping up huge files it can save a little space. Good for archival purposes and emailing
Some ISP's also cap the size of emails you can send that's what ours does so the ability to shrink a large file down by zipping it does come in useful.
When I compress files using OS X's built in "Compress" feature (right-click > Compress) I get almost no reduction in size. Am I doing something wrong here or is this not the same thing...?