Very often the most important thing in developing a screenplay is simply identifying and agreeing upon what needs to change. If nobody on your creative team has experience of shepherding projects through the dark, there's no limit to the number of metaphorical calamities that could befall your script. A script consultant will also be able to point your writer in the direction of not just a better script – also a more marketable film.
Working with screenwriters can be wonderful when communication is clear and a direction is agreed... but when everyone is talking at cross purposes then progress can be slow and it might be useful to bring in someone who knows the demands (and the possibilities) of both writing and producing, can translate the two languages and help set-out mutually-agreed expectations and boundaries.
I'm a big believer in the win-win scenario where everyone leaves the table happy because everyone got what they wanted. And I don't mean compromise – everyone should get what they wanted or better.
Check out this article on some of the ways I can help point a script toward a specific, mutually agreeable, direction http://animalelectricity.blogspot.co.uk/2011/10/fractals.html