Enzymes are evolved to catalyse a particular reaction on a particular substrate with a high catalytic efficiency (k/K, cf. Michaelis–Menten kinetics). However, in addition to this main activity, they possess other activities that are generally several orders of magnitude lower, and that are not a result of evolutionary selection and therefore do not partake in the physiology of the organism.
This phenomenon allows new functions to be gained as the promiscuous activity could confer a fitness benefit under a new selective pressure leading to its duplication and selection as a new main activity.