Creators Forum is a place where Mission Creators and others can release their works. Members of the forum then try them out and provide feedback. The Creators can then fine-tune their offerings and release them to the wider Ship Simulator community.
For the members of the forum, they get forthcoming updates much earlier, and can have an active part in the shaping of what is produced. Many of the in-game Custom Missions are released via the Creators Forum and subsequently submitted to the CMMs for approval. The fact that the missions have already gone through the process means that the CMMs are more likely to approve it.
We also test proposed changes to vessels (e.g. Red Eagle, Pride of Rotterdam, Vermaas and others).
Plus we have some goodies that you can't get elsewhere, such as an 84 page report, detailing every aspect of a large azipod driven Cruise Liner (if you want to know the power ratings of the azipods, as well as which team cleans the toilets on Deck C, this is the one for you!).
Easiest thing is to click on the link in my signature and have a poke around. If you like it, you can apply to join. We have an active membership in the hundreds, so it can't be too bad.