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.
In worship there is always the movement up which brings one closer to the holiness. The higher up you go, the closer to the presence of God and the closer to holiness.
Before the Fall the waters were one dimensional—they gave life. After the Fall, another dimension is added. The waters kill/drown and they give life. This becomes the New Testament language of Baptism.