Overpressuring a fuselage after manufacturing or major repair is one lesson learned from Comet.
If (due to bad design or manufacturing ...) some stress concentration points do exist on the pressurized fuselage, they will be stressed beyond the yield limit during the overpressure test, leaving residual compression stress after the test. Therefore these critical points will not encounter critical tension stress (leading to fatigue cracks) during service.
Problem for comet was that the fuselage section that was fatigue tested for certification was used for the overpressure test before, increasing it´s fatigue life. The serial built fuselages were not operpressured before delivery, so they did not have enough fatigue resistance.