The system allows you to store a lot of information, from the standard fields to custom fields holding anything and everything, but that information is only useful if you do something with it. This is where reporting comes in.
In the reporting section (Located on the left hand menu under "Reports") you can not only run reports on your data but there are static reports telling you all kinds of information from: -
1. How many contacts are not in groups
2. How many contacts don't have valid email addresses
3. How many contacts are missing email address
This means that you can find out all kinds of things about your data