The abundances of the chemical elements (or just abundances, or chemical composition) 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.
The abundances of objects in the solar system (including the Sun) show some consistency, with a pattern that differs from those of other 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. The chemical composition of the universe is determined to some degree and must fit with any such theory, the Big Bang nucleosynthesis and the subsequent chemical evolution, i.e., all the nucleosynthesis since then.
The patterns include ratios between the abundance of elements (abundance ratios). The ratios of isotopes are also of interest: in some cases, spectrography can pick up something, but they are of most interest when the material is available for lab analysis (meteorites and Moon rocks) or can be captured in an instrument such as a mass spectrometer aboard a space probe.