Why were the Acropolis and the Parthenon so important to Athens?
Today we think of the Acropolis as a tourist destination and historical site, but when it was built it served much more basic functions. The Acropolis was built on the top of a hill in order to be defensible, and is essentially one gigantic fortress the size of a small city. (Its name literally means "High City.")
The Acropolis was ruined by the Persians in 480 BC, but was then rebuilt even larger; it was at that time that the Parthenon was built as a symbol of Greece's renewed strength.
In part because of its defensible location, the Acropolis also housed the seat of government of Athens during most of the Classical period, and became an important cultural and religious center as well. Most of the famous philosophers and artists of Classical Greece lived or worked in the Acropolis, and it was there that (what they called) "democracy" was first established as a lasting form of government for a major power. (It was in fact more like oligarchy or aristocracy, because very few people---all of them wealthy men---were eligible to vote; but it was still greater political representation than most cultures had at the time.)
The Acropolis is also notable for the fact that it has stood largely intact for over 2000 years, maintained or repaired by each successive generation. Thus it gives us a direct look into the art and architecture of Athens in the 5th century BC.