Once a film is made, it is left finished, sitting on a shelf somewhere until the filmmaker can sell it to a distribution company.
A distribution company will only buy a film if they think that they can sell it, and to work that out, they ask one fundamental question - who's in it? When was the last time you said, "lets go and watch that film about a difficult but likeable guy who is challenged by someone he normally wouldn't have time for, and as such finds new meaning to life."? You wouldn't. What you're more likely to say is, "Lets go watch that new Jack Nicholson movie."
Without a famous actor it is virtually impossible to get a distribution deal. But it is possible - take 'The Blair Witch Project', the recent 'Cloverfield', 'Open Water' etc. and these were all sold on the fact there was something special about it AND that there would already be a fan base - The Blair Witch project was long supported by enthusiasts on the internet, Cloverfield was 'from the creaters of 'Lost'', Open Water was, well - just plain lucky! The rise in comic book adaptations is no coincidence - they have a guaranteed fan base.
Once I've finished this film, I will have made a feature film entirely on my own - and this is a saleable concept, BUT it's no good if I can't also say, "and I have 500,000 people who are interested and likely to want to see the film."And to say this I will need to know that you are interested ...
- et voila!