Most people think of disaster recovery as a plan that is in place in case of a disaster, such as:
Weather related events, such as floods, tornadoes, hurricanes, and forest/brush fires
Earthquakes
Any of these disasters can disable your primary datacentre and you would have to failover to your DR datacentre. However, most application interruptions are due to more mundane everyday occurrences, such as:
Facility fires
Fiber or communication lines are cut – loss of network
Power failures – outage or sporadic service
Cut power line
Security breach – hacking and/or malicious code
Water pipe breaks in a facility
Human error, such as a redundant system's failure that goes unnoticed
These interruptions can cripple a business if the business does not have a proper DR plan in place.
Another dimension to this point is covered in Chapter 8, Disaster Recovery Techniques for End Users. Think of who is interrupted: sales force, trading floor, executives, or end users.
This may seem like a trivial point, but IT has only so much manpower to dedicate to issues.