If energy cannot be cannot be created or destroyed why does plants need new energy?

Energy is what most people find easiest to understand. But in fact the real issue is entropy. Entropy is a measure of the 'disorder' of a system. Entropy always increases. For every change the total disorder must increase. So plants take up sunlight [reasonably low Entropy energy. ] and convert about 10% of it to low entropy chemical stored energy and loose the rest as high entropy heat. They use the low entropy energy stored in sugars/starches etc to live. [And animals steal it when they can]. So to replace that low entropy energy they must absorb fresh low entropy energy.

Without the concept of entropy, the whole energy discussion does not make a lot of sense. Why can we not just keep re using the energy we buy over and over--- because with every use more and more of it is at higher entropy. High entropy means more disorder, more random less useful. So we buy low entropy electricity, and use it to move a fan blade to move air in a room, the moving air eventually mixes with the still air until all that ebnergy is turned to heat [highest entropy available in the room.]

The energy doesn't get destroyed, but it can be transformed. This is easier to understand with people. We get energy from food. One of things that happens when we exercise is that we transform some of that energy to heat which leaves our bodies. Then it has to be replaced, so we eat again.