If my account is already a member of the administrator group why do some functions on Windows 7 require me to run them as administator? I kind of thought that was implied?
When you successfully log in to a Windows machine, an access token is generated which represents that login session. Among other things, this token contains your username and the groups of which you are a member.
All programs you launch during that session have a reference to this token. When a program wants to do something, it presents the access token to Windows, and Windows uses it to verify the user is authorized to perform that action.
The problem with this is that when you log in as an administrator, every program you launch is running as an administrator. This is convenient but also means that mail readers, text editors, and every random program you download and run have the ability to hose your system if they want.
Enter: User Account Control
Beginning with Windows Vista, when administrators log in they are assigned two separate access tokens:
During normal usage, the standard access token is used. When a program is launched using this token, it has the same rights as a standard user. The moment the program tries to do something requiring a privilege not given to standard users, Windows shows the UAC dialog asking if you want to grant administrative privileges. Once you confirm, Windows replaces the program's access token with the elevated one, then allows it to continue.
The purpose of UAC is to inform the user when a program is taking advantage of their administrative privileges. Text editors and mail readers shouldn't need to run as administrator, so seeing the UAC dialog appear for these programs should be cause for alarm, or at least some scrutiny.
Microsoft's explanation of UAC and some steps for dealing with it are available as well.
The idea is to provide least privileges.
The principle of least privilege is widely recognized as an important design consideration in enhancing the protection of data and functionality from faults (fault tolerance) and malicious behavior (computer security).The more programs are run with administrative credentials, the more vulnerable you are to faulty or malicious software. The compromise that allows you to do things that require administrative access without doing all actions with that level of access is to explicitly prompt on a case by case basis - just because you are in the administrative group does not mean you want to provide every program you use with full administrative powers.