Why sign a jar?

I was recently asked, why would you ever need to sign a jar? What advantage does it give me?

I would like to put this question to the Java.net community: have you seen a need beyond allowing your applications to run with Java WebStart, or perhaps within the confines of an Applet.

Endorsing jars allow youto over-ride classes, and is something I have seen used in the Java Web Services Developer Pack, however the reasons to endorse a jar might often be quite different to why one might sign a jar.

So over to you... I am really interested to hear about used of signed jars that I may not be aware of...