What exactly is the idea behind the charge clamp?

The idea of the diode clamp is that you can enforce a DC offset with a diode. If you are driving a signal through a capacitor, then you could have a DC offset according to how much charge was on the capacitor when it was first connected to the supply (total voltage at $V_0$ is $V_s+V_{\rm cap}$, where $V_{\rm cap} = Q/C$). This could be some unpredictable value, and might be undesirable. To get a specific offset, you can put a diode across the output. The diode would only conduct when $V_0$ on the right hand side of the capacitor is negative. When $V_0$ is positive, current can't flow through the diode and charge is trapped on the capacitor (positive charge on the right hand side). With positive voltage on the right hand side, the diode never conducts and the charge on the capacitor can never swing negative. So what you end up with is a DC offset at the output. (BTW you could change the offset by putting a reference voltage, e.g. DC power supply, in the branch with the diode.)