A. It is the address used by default as the local address for broadcast and multicast packets sourced locally and sent out of the interface.
B. You use the primary address when you have multiple IP addresses belonging to the same subnet on the same interface.
C. It can be useful for selecting the local address used for packets sent out of unnumbered interfaces when multiple non-127 addresses are configured on the loopback interface.
D. By default, the primary address on an interface is selected as the numerically highest local address configured on the interface.