Sharing (output): This is content that you publish. Depending on your real-life relationship with your contacts, the information you share will not be the same for your different social circles. At the same time, your social circles may overlap. This means one person may be a part of two different circles at the same time.
Streaming (input): This is content that your contacts stream to your Google+ homepage. Streaming is different from sharing in the sense that streams depend more on your interests rather than your relationships.
Hence, to achieve more control over your circles, I believe it is necessary to divide them between sharing and streaming.