The only way to preserve them is not to use them, but it's more sensible to go on using them (which we will do come what may), but to use them more economically, and to use renewable energy where...
It is important to conserve nature for future generations because, if we don't conserve the future generations won't have a chance to see and expirience nature as our generation did.
Because is essentially an endless supply of energy, with only a small percent of the actually total that even reaches the earth. Cosmically speaking the sun will be around for a few more billion...