Why would I put an ID on a script tag?

The one use I've seen of this is if you want to provide widget for customers and you instruct them to place the <script> tag wherever they want the widget to show up. If you give the <script> element an ID then you can reference that inside of it to place the code in the right place. That's not to say that it is the only way of achieving that, of course, but I've seen it done and suggested it in the past.

I've seen it used for Microtemplating, where you can put a template in a script tag and then reference it through the ID.

Here's great post with javascript microtemplating by John Resig - note that this is not the ONLY way of achieving this, only Johns version of it.