To make it easier to flag problems in your Chef cookbooks that will cause Chef to blow up when you attempt to converge. This is about faster feedback. If you automate checks for common problems you can save a lot of time.
To encourage discussion within the Chef community on the more subjective stuff - what does a good cookbook look like? Opscode have avoided being overly prescriptive which by and large I think is a good thing. Having a set of rules to base discussion on helps drive out what we as a community think is good style.
Foodcritic has two goals:
More at http://acrmp.github.com/foodcritic/
And BTW, why not also run tailor, a static code quality analyzer for ruby language ?