Cristae ("kris-tee") are the internal compartments formed by the inner membrane of a mitochondrion. (The mitochondria exist in cells and primarily generate energy.) The maximum surface for chemical reactions to occur is within the mitochondria. This allows cellular respiration (aerobic respiration since the mitochondria requires oxygen) to occur. The cristae greatly increase the surface area on which the reactions take place. If they were absent, the inner membrane would be reduced to a single spherical shape, and with less reaction surface available, the reaction efficiency would be likewise reduced. Therefore, cristae are a necessity for the mitochondria to function efficiently. (information from wikipedia)