Magnesium deficiency doubles the likelihood of systemic inflammatory disease (of which Crohn's and Ulcerative Colitis are examples). Severe magnesium deficiencies are more likely in people who have Crohn's disease or other conditions that affect digestion as magnesium is absorbed in the GI tract, (more so in the small intestine than the large). Where inflammation of the gut is inhibiting absorbtion, it may be advisable to increase the intake of magnesium either in naturally occurring sources or supplements. I know that many of the natural sources may be on the absolute "no no" list for many Crohnies and Collies, but do check before diving for the bottles and always take medical advice.
Here are a couple of articles that provide more information on the importance of magnesium, sources and absorption.