If you extract energy from a substrate (e.g. glucose) step-wise, a lot more free energy is released that can be used. However, if you were to do this in one single step, very little useful energy could be harnessed. Why is that?
I think there are two relevant points here.
1) explosive reactions can give significant useful energy - e.g. internal combustion engine
2) step-wise processes may go slower and release less heat / light other energy that is not so useful. This is a bit like in thermodynamics you get maximum useful energy from slow reversible processes.