For three reasons:
External Risks
External threats seem more visible and frequent.
Every month announcing new security vulnerabilities.
Growing requirements to protect the reputation of the company
User service levels
The business is increasingly dependent on IT.
The applications are becoming increasingly critical.
The high availability becomes a differentiating factor.
Operational Complications
24x7 Operations reduces the backup window and maintenance.
Merges and consolidations added presure on centralized IT systems.
The frequency of changes will increasingly impact business processes and IT.Key Aspects
In order to ensure the continuos operation of a bussines in the case of a
very long server interruption every plans, resources and actions are
used whether it is planned or not.
Bussines continuity
Disaster recovery 
Disaster
Tolerancia a fallos (Fault Tolerance HA) Disaster Recovery
The
ability to continue operating through Plan for the recovery of
hardware and software design the environment when
which allows the computing infrastructure a damage ocurres
to prevent failure
