Whereas normal energon provides the fuel that keeps the Transformer race rolling along, Rarified Energon is something more. In this form it is the base material of all Transformer life. Rarified Energon is the base of the Transformers physical forms AND the stuff wich Sparks are made of. Rarified energon is also, apparently, of limited supply throughout the multiverse. If it all were to be used up no more Transforer life would come into being. Given this its protection is of vast importance to Primus, and he as tasked one of his Thirteen original Transformers with its protection.