I can't answer for how Google would like you to use Circles, but what I can tell you is how I use circles.
I have circles for friends, colleagues, colleagues by company (as a contractor I move around quite a bit), people I know purely via G+, other UX people, other UI people, other Developers and interests such as Photography, Mountain Biking and Travel.
The great thing about the way in which circles work is that you don't have to just send things to all or one. If I have something to post relating to UX I can send it to all colleagues past and present, UI people, UX people and Web people. If I have something about photography I can decide whether only other photographers would want to hear about it or if it's something I should post publicly to be enjoyed by all.
Like I said that's just me, and how I use them. If this [personal opinion] isn't the kind of thing you want to know maybe rephrase the question as mentioned in the comments! :)