We have to convert it into another form to make it work for us. If you dam it and install turbines, you get electricity. If you burn it, you can warm your home, cook food or raise steam in a boiler - to make electricity. Life needs energy. Life's energy is self-perpetuating only so long as there is sunshine.
Animals eat the plants or each other, die and decompose, freeing nutrients for plants again.
It is a sustainable cycle. For hundreds of thousands of years, humans have merely been a part of this cycle. Then we got clever and discovered that we could use energy other than from our own muscles to do work for us. First we discovered that wind could drive our ships and water could power our mills. Later, we found that coal could make heat and steam for machinery.