A glaze is often used to fuse/blend colours together,
Say you base coat a cape dark red, highlight it with a slightly lighter red then enhance highlitght the most raise parts.
A red glaze over this would help to blend these together and stops or breaks up lines that can be cause from not blending properly.
I hope you get what i meant.
regards Robbie.
I make my own using vallejo glaze medium
heres a link to see how it works.