The purpose of this chapter is to provide for an actuarially sound system for the payment of annuities and other benefits to officers and employees and to beneficiaries of officers and employees of cities and towns thereby enabling such employees to provide for themselves and their dependents in case of old age, disability and death, and effecting economy and efficiency in the public service by furnishing an orderly means whereby such employees who have become aged or otherwise incapacitated may, without hardship or prejudice, be retired from active service.