The combination of the Validation Type and Priority selected on the Configurable Validation table determines the flow and controls execution of validations. Precondition comes in play when a large set of validations are defined on a business object and belongs to a common condition that can be checked as a precondition before evaluating validations. This option is very useful and avoids negative performance impact by allowing execution of validations only when necessary.
The following diagram depicts the control flow and use of preconditions.