Why do Electrical Engineers need to do short-circuit or fault calculations?
The answers are simple. First, to determine the rating of the electrical equipment to withstand thermal and electromagnetic effects of the short-circuit current. Second, to determine the settings of protection equipment.
In the first usage, the peak short-circuit current is used. This current occurs about 1/4 cycle after the initiation of short-circuit current. Electromagnetic forces are proportional to the instantaneous value of the current which is why peak short-circuit current is essential in the fault calculation. This will determine how much support or bracing is required for a switchboard. This switchboard rating, i.e. 62kA @ 1s, is calculated.
The RMS value of the decaying short-circuit current heats up the conductors due to its resistance. The RMS value of the short-circuit current in combination with the total fault duration determines the thermal effects of the short-circuit current.
In the second usage, the minimum fault current is used. This is needed for the design of the protection system and the settings of the protection relays.
The bottom line is actually safety.