(Snippet from an email to a client who asked if they needed to download their developer certificate.)
The developer certificate is what lets a developer build an app on their computer and put it on their device(s) directly over the wire, without messing around with ad-hoc, installation, etc. So, when I do programming -- whether for you or someone else -- I just leave an iPod plugged in to my computer and, every time I compile, the app just shows up over there & I can test immediately.
If you wanted to compile your code and use it directly, you'd need your developer certificate, otherwise you don't. I don't need yours because I have my own.