This is because power systems are susceptible to faults and transients. Protective relays are designed for prevention from injury of operators and to protect the equipment. There are actually several case studies found in the internet regarding such matter. You may read some of it for further knowledge.
Also, since power systems usually have more than one generating plant, isolating the faulted portion of the line can reduce the impact of the fault by allowing power to be re-routed around the faulted portion while allowing the repair crew to work on the line safely.
This falls under the topic of coordination or selective coordination. When you have multiple OCP devices in a circuit, say a Switchgear feeder breaker feeding a Load Center Panel feeding a Branch Panel, you use the curves to insure that the breaker closest to the load trips openrather than trip open a breaker upstream. Generally, you can get three series breakers to cooperate and no more because of curve overlaps. When the curves are uncertain or there is too much "slop", or you are looking to trip open on something other than over-current, then you use relays driven by special sensing devices.
