The fossils give us the opportunity to study the physiology pf the species that existed in the past, and through their fossils comparisons can be made with the species that exist in the present.
By comparing them, lot of species can be connected with one another as they share common characteristics. By comparing the species that exist in the present, and slowly get back in time with the species that are extinct with whom they share common characteristics, it can easily be seen how the species developed, evolved, and what caused that.
Nice example are the felids. They all share the same basic characteristics, be it in the present or in the past. Through time they developed some specific characteristics on top of the basic ones, depending on the environment, so the connection is easily seen that they are related, but also that some factors contributed that they become bigger, smaller, nocturnal, day dwellers, had sabre teeth, longer legs, shorter more muscly legs etc.
