2) For emphasis – the clean-cut edges will stand out against most backgrounds, plus you can accent the edges for even greater impact.
For this one I used the little blue heart to bring that colour across the page and unite the two blue areas. The larger green heart I used to balance out the stamped figure, and I outlined it in purple to bring that colour into the bottom right, which helps to draw the eye around the page.
3) To add an actual tactile element to your design, even over already printed, painted, or stamped texture.
Going over the kraft card die-cut moon with silver gel pen both ties it in with the stencilled silver stars and makes it stand proud of them. Adding the little kraft card heart echoes that material, and the tiny black one repeats the one in the stamped sentiment.
4) To create patterned backgrounds instead of using printed papers or stamped collage.
Here the large distressed hexagon die adds all-over background interest and holds up even over the floral stamped section. I inked it to coordinate with the other design elements on the page.