Why the Death of mive stars...?

You seem to know what causes supernovas and why they exist. But the reason they are important is because in the big bang, the only elements produced were hydrogen and helium. In stars, fusion causes the creation of heavier elements (up to Iron on the periodic table). However, when stars do not explode, these heavier elements stay locked up inside the star and after that, the resultant white dwarf. When stars supernova, the heavier elements are tered, including those needed for the creation of life. The mive amount of energy produced in supernovas also causes the heaviest elements (Iron and up) to be produced.