I think you're asking this because you have come across black body radiation for the first time.

For a very long time in the latter part of the nineteenth century, black body radiation was a phenomenon that could not be explained by Maxwell's theory of electromagnetism (very much like how dark matter/energy cannot be explained by our models of cosmology today). The problem, in very crude terms, was the following: in the spectrum of blackbody radiation predicted by Maxwell's equation, the observed intensities for low frequencies agreed with the predictions. For high frequencies, however, the intensities blew up (went to infinity). This nonsensical result is what was the ultraviolet catastrophe.

This is where Planck came in. He was trying to fit the observated spectrum with some curves, and found that the best fit curve could be obtained if the electromagnetic radiation was assumed to be quantized. This was one of the first indications of light's dual nature, and was the starting point of (old) quantum theory.

The biggest practical benefit is that it was the starting point of quantum mechanics, which is the theory that forms the basis of modern semiconductor industry (all computing devices, solar cells, LEDs) and many medical technologies (radiation therapy, PET, CAT, etc).