Why do ruminants digest their food in two steps...?

The ruminants (cattle) do not have incisor or canine tooth. Thus they only tear and swallow the foliages like grasses or leaves but cannot chew properly. The swallowed food goes to the first chamber called rumen. Here the food gets partially digested and transferred to the second chamber. From this second chamber the partially digested food again goes back to mouth. Due to half way digestion the food matter becomes softer than earlier and the animal can chew it well. Again they swallow the chewed food for the second time where further digestion is carried on. Thus ruminants digest their food in two steps as they swallow the same food twice.