You Are What You Eat:

While true that CrossFit is a core strength and conditioning program, in order to elicit as broad an adaptational response as possible (the primary goal of CrossFit), your diet needs to be in order for at least a few reasons:
1) Without a proper nutrition program, you won't be able to work out at the intensity level required for achieving optimal fitness;
2) Without a proper nutrition program, your recovery from workouts will be hampered;
3) Finally, with a proper nutrition program, you can prime your hormonal environment for maximal adaptation (i.e., the response to exercise will be far more profound).