Lets say I have circuit with resistance 4Ohms and the current 100mA. If I use Ohm's law, I get 0.4V. Why is it important to know the voltage if there is already a current? Also, when you know the energy available per unit charge (voltage) and current, I can calculate the power. In that case, wouldn't it be useless to know the voltage when you aleady know how much energy is being used (power)?