The sort of description science strives for is not just a catalog of events but an explanation of those events. We want to understand why things happen the way they do. To achieve this understanding we start by observing regular patterns of appearance. For example, every year as the days get longer in the spring season, the temperatures rise and trees grow leaves. Noticing this regularity does not yet by itself yield an understanding of the causative relationship between these events.
Scientific experiments are a powerful procedure for investigating causal relations.
If instead we observe occurrences of C only after spontaneous occurrences of B but never after the occurrences of B that we freely generate, then we can be confident that C is not caused by B, but instead caused by some prior A that also causes B in a parallel chain. Note the complementary relation between free will and causation revealed by examining the experimental method. Curiously the scientific description of the world does not include the free will of the scientific experimenter, even though that free will is an essential requirement for the generation of that description. It's a bit like the blind spot in the visual field caused by the connection of the optic nerve to the retina. The optic nerve is an essential requirement for the effective functioning of the retina, yet the retina is blind just at that point.