The relief valves are there in case the outflow valve doesn’t work properly.
If the outflow valve stuck shut when you were climbing then delta P would increase. Without a pressure relief valve the delta P could get a level that could damage the airframe.
That could be a reduction in fatigue life or in extreme the fuselage could split open like a sausage.
There is I think an example somewhere of a ground test of an aircraft on the ground which they pressurised to test the PRV. The PRV didn’t work and the plane was written off.
Similarly if the outflow valve stuck shut or nearly shut in the cruise you would get a negative delta P as you descended. This hopefully would open the negative pressure relief valve or else the plane might end up like a crushed drunks can.