Given the current financial situation of the USPS as well as the declining revenues associated with first rate mail delivery, allowing the USPS to branch out to other sectors such as banking which can take advantage of the large number of retail outlets it has available to it would allow it to maintain solvency.
Allowing the USPS to get into banking has the additional advantage of allowing the majority of Americans to have a place to save their money without the risk that the large banks with investment arms would wipe out their savings during financial crises.