Some people ask me about this, so this is posted here:
Link: The $100 Code Signing Key
The optional $100 code signing key simply permit more flexible development such as programmatically modifying PIM, bluetooth programming, program control of the backlight, BlackBerry audio programming, programatically dialing numbers, loading/saving settings, etc. The key is unlimited use; you can use this key as often as you want for as many applications you want, released under your name or your company name.
It is a cryptographic signing key given to you by RIM, that "unlocks" a lot of programming features.