Why do I need to give all this information to submit a plugin or script?

The easy answer is for verification purposes. Any submission that comes in needs to be reliable and trustworthy. Also, for protection. The content needs to be validated and checked before placing it into the archive. Providing contact information is crucial in case admin has questions about your content, before uploading to the archive. 

Can I request a script through GimpScripts.net?

All script/plugin requests should go through a GIMP site, like GIMP-Forum, GIMP Learn, or GIMP Chat. More coders frequent those sites and your odds are improved at getting your request accomplished. See GIMP Pages for links to those sites.

What if a script or plugin doesn't work? Who do I contact to report it?

Both are good questions. Using  a text editor, you can open any scripts or plugins, and at the very top of the code should be a comment section that lists the coder's name and email address. Like the example below:

If you requested the script from someone on one of the GIMP sites, you can always refer the broken script to them. Always read the instruction files that come with the scripts (if they come with them). Often times, a script or plugin isn't actually "broken", and you're not meeting the conditions of running the script, like opening a certain size image, etc.