Finches have different shapes of beaks for they adapted to their environment. By definition of the Evolution, established by Charles Darwin, he states that 'animals (in this case Finches,) will either: 1) produce more than is enough in the hopes that some will survive, 2) Adapt to their surroundings, or 3) Create changes (or positive & negative mutations) to try to survive. These examples were found during his trip in Galapagos Island, where he observed that in different parts of the island, bird beaks have transformed. Places with hard-exterior nut shells, generally saw to that the bird beaks were shorter, but stronger and larger. Places softer food, saw the bird beaks being longer, but smaller in size. Essentially, the Finches beaks have different shapes, to adapt their "tool" (their beaks) to their surrounding. A very poor (but what can work) example of how this works in human standard, is like having different tools for one job. While only having one tool may be able to help solve most problems, having different amounts would be great in solving problems with ease.
However, it is important to note that Darwin's Theory has it's own fair share of problems. It is a theory, in the end, not a law. There are times where species are seen to evolve when placed in different situations, but afterwards, when placed back into their natural location, to failed to keep the "adaptation". Traits that are created are not random (as suggested by Darwin's Theory), in which they keep what helps them survive, but they generally go through generations of trials and errors, meaning that every step of a mutation is a trial of adaptation to what is survival. There is no randomness in the world when it comes to survival, only the will to survive.