Why does your spectrophotometer have to measure different wavelengths?
Different compounds interact with light of different colors (or wavelengths of light). A compound may absorb one or more colors strongly while not absorbing other colors. By looking at the spectrum generated from such compounds, one can begin to idientify different components or at least classes of components. And by using several different types of spectroscopy, one can get more detailed information as different types of spectroscopy can give different information. Raman spectroscopy would be such an example in that it yields different but very detailed information to help identify compounds by their structure.
UV-Visible-NIR Absorbance Spectra of multiple layers of Graphene