I have seen helical wheel projections used to illustrate amphipathic helices in proteins. Are there any other uses for these models?
Helical wheel projections can be useful anytime you have a protein or peptide with one or more α-helical regions. They allow you to see whether any "property" is present along one side of a peptide. This can be useful in recognizing potential protein-protein interacting motifs such as leucine zippers.
The wikipedia article on "Helical wheels" also looks like a good place to start learning more about this.