A Orchestral Mockup is an extensive demo of a recording project built using virtual instruments.
Orchestras are usually reluctant to play music without first knowing it. An orchertral mockup will help musical directors decide whether to include it in their repertoire or to release it. It can also help the musicians and the conductor to rehearse.
These demos are often used on projects that require large budgets to record, such as soundtracks. A orchestral mockup allows you to listen to your compositions in an environment that approximates their final version, allowing you to approve or modify the project before the budget for recording live musicians has been committed.
An orchestral mockup will help a composer a lot to know what his work will sound like in an orchestra. Having a real orchestra available is very expensive, a well-made orchestral mockup can sound very close to a live recording and will be a much cheaper option for a composer.