typical application of an HRC fuse, why would this application need an R rated fuse?

Answer 1

From Littlefuse:
"“E” Rated fuses are considered to be general purpose fuses and can be used to protect against low and high values of fault current. “R” rated fuses are designed for back-up protection.
They must be used in series with other devices, such as motor overload relays, in order to achieve both overload and short circuit protection.

“R” rated fuses provide required short-circuit protection for medium voltage motors, motor controllers and associated circuitry. These components have limited ability to absorb the
energy of large short circuit currents. Medium voltage motor controllers contain overload relays which provide both overload protection and locked rotor protection to the motor. The
controllers are also intended to interrupt low value short circuits within the capability of the motor controller. This protects the medium voltage fuse from sustained overcurrents which are less than their minimum interrupting rating.

NEMA Standards for “R” rated medium voltage power fuses require that they operate within 15-35 seconds when subjected to an rms current 100 times the “R” rating. For example, a fuse with a 2R rating will open within 15 to 35 seconds on an applied current of 2 x 100 = 200 amperes."

Answer 2

Hrc Fuse