Causal powers: what are they? why do we need them? what can be done with them and what cannot?

What are causal powers and why should we believe in them? Causal powers are now a central topic in metaphysics but my defence of them does not begin there, but rather in studies of the practices of the sciences, especially in my case, of physics and economics. Both of these use the analytic method: they ascertain the behaviour that would result from the operation of a cause ‘in isolation’; then take this behaviour to provide the ‘contribution’ that that cause makes to the behaviour that occurs even when the cause is not in isolation. What counts as ‘isolation’? And how is the notion of ‘contribution’ to be understood?