It may just be a habit like chewing gum or compulsively eating. The ice is certainly not as bad as munching on sweets all day long but unfortunately dentists will tell you chewing ice is really bad for your teeth,
This is actually a form of Pica and it is due to nutritional deficiencies that give the body strange desires to eat or smell non-food items. Ice won't hurt you that much, hard on the teeth. Some people are drawn to detergents and all kinds of strange things. My friend had the ice chewing Pica throughout both pregnancies and it ended when her babies were born.
I never knew it was a form of pica...that's new information for me. There may be any number of reasons why someone chew on a cup of ice: the crunch; moisture due to dry mouth, (some people take medicine or have medical treatments in which their saliva glands have been affected); soothing nausea, (I had vertigo with nausea very badly and this helped immensely); giving up one habit and substituting this as a tool to stay free of the habit, (i.e. smoking).
And, some people enjoy chewing ice. Hope this gives you info to 'chew on'.