The first question, "which reactions are occurring, making which species?", is important in many fields of chemistry, not just kinetics. For example, if one wants to synthesize a molecule, one needs to be able to predict what the major products from combining a set of reactants, reagents, and catalysts at certain reaction conditions will be. But the question is particularly important in kinetics, since to build a kinetic model we need to predict not just the major products, but also the important reactive intermediates and reactions, and the important minor byproducts (e.g. those that are toxic, or which cannot be allowed to contaminate the final product). And ideally we would like each reaction to be elementary, so we can predict how its rate depends on concentrations.