Why State retention (Retention) is required in Power-Aware Design ?

• State retention is the ability to retain the value of a state element in a power domain while switching off the primary power to that element, and being able to use the retained value as the functional value of the state element upon power-up. 

• State retention can enable a power domain to return to operational mode more quickly after a power-down/power-up sequence and it can be used to maintain state values that cannot be easily recomputed on power-up. State retention can be implemented using retention memories or retention registers. Retention registers are sequential elements (latches or flip-flops) that have state retention capability.