Isaiah first reminds the people of who they are and then tells them why they are who they are; to bring the teachings and promises of the LORD to all people.
This food, already purchased and freely given in our pericope, is a foretaste of the feast to come as well; the marriage feast of the Lamb in His Kingdom which has no end.
To dwell with a Holy God in their camp, Israel lead holy lives. Anything standing in the way of and threatening this holy relationship must be avoided or eliminated.