In many cases, the quantum yield is the hard limit, hence it tells you, better than a power yield for instance, how far a process is from the best possible yield. This makes it also a tool for understanding.
Take for instance visible light on a silicon photodetector. You get almost on electron photocurrent per incident photon, which is the limit. It explains you why amps of photocurrent per watt of light drop at shorter wavelength: just because a joule of blue light contains fewer photons than with green light. Having checked the quantum yield, you know that competitor detectors working similarly won't do better.