Try putting two glazes side-by-side to produce detailed pictures then repeat with under-glazes. Fire both and see the difference. Under-glazes do not move or blend with each other. Most glazes (I know not all) will bleed into each other, or one will move over the other, or they will slide slightly down a vertical surface.
For me, under-glazes are for when you want more than one colour on a piece, glazes are for single-colour or "doesn't matter" if they run together.