Fossils are an important piece of proving the theory of evolution because they present the physical proof of its' existence. By discovering fossil evidence of a creature from varying ages, reference points are present that allow us to see the physical transformation that has occurred through time.
I never forget one of the first times I had gone to the Royal Ontario Museum as a child in primary school, and got the chance to visit the dinosaur exhibit which was undoubtedly one of the favourites of the entire class. This included a lot of fossil evidence as well as reconstructed models of dinosaurs, and even at a young age I could see that there were similarities between creatures of old and those that live today.
Some of the more obvious examples were the Wooly Mammoth, which we could tell was related to modern day elephant, and a creature called Brontotherium that looked a lot like a wild boar, and the Saber Tooth Tiger that closely resembled the large cats of today.
The more we looked around the exhibit, the more we realized it was like a huge puzzle that detailed the natural world from long ago, but which was still connected to our own in many ways.
And as in any puzzle, the more pieces you have, the bigger the picture will be. Today, there is an impressive amount of fossil evidence from all over the world, whether it be in parts of Kenya, China, Canada, Australia, Argentina, or the United States. This evidence is examined and studied by paleontologists that allow them to deduce in a scientific manner what the world would have been like during the specific point of that creatures' existence.
For those that would dispute this physical evidence as being unreliable or misinterpreted, that can be said about anything and everything. In all sciences, there are always detractors that do not buy into or believe what has been laid out before them.
We still have some that don't believe man has landed on the moon, although there is enough photographic and video evidence to support the fact that this event did take place. There are those that feel that D.N.A. testing is unreliable and not as accurate as science would have us believe, although it has been proven time and time again to be a very exact science with real substance.
Without the existence of fossil evidence from around the world, the evolutionary theories that exist would lose most, if not all, credibility. Everything in this world is conjecture, unless you have the physical proof that substantiates what you're proposing. Fossils are a clear and indisputable window into the ancient world that allows us to compare life forms and see a definitive pattern of evolutionary change that has existed throughout time.