The functional groups determine the shapes of macromolecules and this in turn determines their functions.
Functional groups affect the bonds that hold a macromolecule together. For example, the ring forms of glucose in starch and cellulose have different configurations. As a result, cellulose and starch serve different purposes.
Functional groups also affect the polarity of the bonds. Lipids consist of hydrophilic heads (red) and hydrophobic tails (black and white). This characteristic of lipids allows them to form bilayers, which serve as membranes of cells.
Proteins have complex structures caused by interactions between the functional groups. A change of a single amino acid can make a major change in the function of a protein.