The abundances of the chemical elements (or just abundances) are of interest in astrophysics, i.e., to match astrophysical theories to observations. Abundances in stars, planets, moons, and the interstellar medium can be worked out with some reliability through spectroscopy, and study of albedos, using knowledge of basic physical and chemical processes.
There is some consistency in the abundances of objects in the solar system, including the Sun. These differ from those of many observed stars, suggesting that individual stars and their systems inherit abundance characteristics from the material that formed them, e.g., of specific molecular clouds. Given this conclusion, any divergence from the usual abundance seen throughout the solar system (e.g., the Sun's lack of lithium or Earth's lack of hydrogen and plethora of oxygen) demands reasonable explanation.
Theory is called upon to explain the observed abundances of the solar system (and those of other stars) based upon that of molecular clouds, and those abundances, in turn, on nucleosynthesis such as that within stars and supernovae, and so on, back to the Big Bang.