Read all about the benefits available here: https://developer.apple.com
If you're going to develop for iOS (for example), you can run your apps on an actual device while the free account only lets you run apps on the simulator.
If you want to sell your iOS app in the App Store, you must join the paid program.
What is the difference between (this site) Apple Support Communities and the Developer forum.
Aside from any other benefits of the paid developer programs, the Developer forums are focused solely on developers. This forums is just one little out-of-the-way corner of a much larger, more consumer oriented discussion site. Questions asked in the Developer forums are typically more difficult and may take several days to get an answer. Questions here are more beginner-type questions and you will get an answer much more quickly. The Developer forums are mostly populated by professional developers who have a much different perspective than you get on most other public internet forums. If you don't know what I'm talking about then you probably wouldn't believe me anyway.