There are Communities for every game out there. They pop up on Reddit, they are on dedicated fan sites, wikis, official websites, Most are online only but some engage in face to face meetings and conventions. Loving a game and finding others who share your passion can be very rewarding. There is a downside to this too of course, as Community forums are notorious hotbeds of troll activity, but if you can rise above that you can add to your experience of the game.
Dreams needs an active community because the game is just a ball of Playdoh without it. Not only will cooperation be required to make anything of worth, but the game allows you to make collections and share your favourites. Sharing techniques will be a huge part of the community forums for this game, and there will be a lot of people needing help from those who learn quicker and have better understanding.
I do not know if Media Molecule plan on having an official site and forum for this game. The lack of visible activity from the Community Manager at the moment is surprising. Maybe dreambubble.me will rise as THE Dreams forum. This would be great because I am already engaged in this community. Either way the community will need to be active to promote Dreams within the community itself as well as outside. I would like to see people promoting their favourite levels, making tutorials, making compilation videos, talking about development of the game. If the forums are just there for people to moan about bugs it will be very sad.