They help in that, many recently discovered fossils form series that trace the evolution of modern species from extinct ancestors. Patterns in the distribution of living and fossil species tell us how modern organisms evolved from their ancestors. Evolutionary theory explains the existence of homologous structures adapted to different purposes as a result of descent with modification from a common ancestor.
Theory because a theory is a well substantiated explanation of how a particular phenomenon works based on the set of facts which had been repeatedly confirmed through observations and experiments. A scientific theory gives reliable account of a phenomenon.