Why should we use some resources like coal and petroleum in limit?

As we know Coal and petroleum are fossil fuels. The dead organisms take millions of years to get converted into these fuels, On the other hand the known reserves of these will last almost a few hundred years. A part from this, these products are not environmental friendly as burning of these fuels are the major cause of air pollution and their use is also linked to global warming therefore, we should use these resources only when it is actually required.