Fossils tell us about life long ago. More than just a curiosity of nature, in the proper context, a fossil can tell us about environments and organisms of long ago. We can learn about the climate and weather. One can decipher current flow in ancient oceans or streams by the alignment of fossils. Fossils give us insight into the development of organisms.
Evolution is the process by which life changes over time.
This process is almost immeasurable over the life spans of a handful of human generations. A million years is needed for a species to change to a new one. Other fossils are widespread. However rarity does not always play a role in the value. Complete dinosaurs are rare.