From a business's perspective, the MIT license is similar the BSD license: you can incorporate MIT licensed code into commercial products/proprietary code and only have to include copyright notices. GPL has source code disclosure implications which can be deal breakers for some businesses.
I think he was asking why anyone would choose GPL, considering MIT is more permissive in every way.
Making something dual licensed allows for the next dev to make modifications and then release those mods under his/her preferred license. You could make a mod of favicon.js and release your mod under GPL to ensure any further modification of your code and your favicon.js branch stays under GPL.
It's possible to release a mod under GPL when the original license is just MIT.