An idol is a field (I guess it can be known as a "field") that has many different routes.
A girl can have a passion for music and singing starting from a young age. She can be influenced by a particular group (as some from Momusu and AKB have stated).
Idols can go in many different directions - modeling, acting in movies/dramas/plays, voice acting, solo artist, announcer, tv talent - endless possibilities. A way to get into a particular field is taking that audition chance for an idol group.
For instance:
A girl gets into an idol group. She might not sing or dance well, but she practices and learns techniques and skills to better herself. Meanwhile, her group actively performs in front of an audience. The group gets CM opportunities, and her face can be seen on national tv. The next year, she gets scouted by a tv drama for a minor role. The year following she's lead actress in a movie. Some time later, she decides to leave her group after finding her passion for acting.
There are girls out there who want to do it, literally, just because. It's fun and exciting and as a group they tend to travel to perform. Some idol groups are local, and could bring hope and happiness. Such as when the earthquake and tsunami devastated the Tohoku region.
For some girls, it's a huge chance just to get their name out there. A popular agency can look at her previous experiences and choose her out of a handful of others simply because she has acting experience and can present herself well in front of a camera.
So, for 9,000+ to audition for one group, it's really something.