networking - What is the purpose of the "Network IP address", why can it not normally be used?

Maybe a noob question but when referring to IP subnets, what is the purpose of a network IP.

i.e. with a network like 192.168.1.0/24, you can't normally use the .0 for a host address. Likewise the .255 is assigned to the broadcast. This I understand but the .0 I do not. What is the purpose of it and why are point to point links with /31 mask bits able to do away with it?