How are Galapagos Finches a good example of natural selection?

because they show how different environmental stressors can influence adaptations/mutations/variety in population which then allow natural selection to influence which species survive and reproduce better and thus lead to evolutionary changes.  Keep in mind that natural selection acts on the individual and evolution acts at the population level. Hope this helps.

Natural selection effects, therefore, are measurable in these finch populations. This research is rightly regarded as a significant contribution to ecology and biological variation. What can be learned?
Selection pressures act on characters of organisms which have relevance to survival. These characters may be hereditable.
In a relatively stable environment, changes in characters will oscillate about a mean. It is possible that, with selection pressures acting in a consistent direction, the population characters will shift so far that eventually a new sub-species or a new species will be formed.
Natural selection is not a creative influence on organisms. It can only act on and mould what is already there.
With regard to point 3, neoDarwinism links natural selection with sources of genetic variability (primarily mutations) to inject creativity into their vision of origins. The research with the Galapagos finches has cast no light on this aspect of evolutionary theory. No mutations are involved in the observed changes. There is no new genetic information. The research has added to our knowledge of ecology and biological variation, but it does not provide any support for any theory of large-scale evolutionary change.
From a creationist perspective, all finches derive from a handful of created kinds: interbreeding studies show that the finches form clearly-bounded families with hybridisation taking place within the family groupings and none occurring outside them. The field observations on Daphne Major are entirely consistent with this interpretative framework. There is no reason to doubt that speciation within an ancestral finch population has taken place over time, and that natural selection has played a role in moulding their descendants into the different genera and species that we see today.

When Darwin collected these birds, he had no idea they were all finches, because these birds occupy a wide variety of ecological niches that are not normally occupied by finches elsewhere.  He showed these birds to a bird expert and that is exactly what he was told. That means these birds must have had a common ancestor but they changed in order to exploit new sources of food.  They show Darwin that when the opportunity arises, a species can adapt and change to exploit a new way of life.  A species changing through time, which is what creationists do not expect since they claim that all species were created by god and are immutable, is what evolution is all about: one species changing into another, different one through time and natural selection.