Why is coffee beans used to smell test perfume?

Coffee bean smell creates an olfactive break. After smelling several fragrances the brain gets saturated and as a consequence your olfactive sense is considerably reduced.

Smelling coffee bean will "distract" your attention from the fragrances for a moment and then allows the brain to reset its capacity to analyse fragrance.

The effect and mechanism is more psychlologycal than a molecular reaction created by the coffee.