Designing and building ETL layers of QVDs can pay dividends as your use of QlikView grows.
Always try to plan your use of database tables in advance. It's usually much better to start by devising your QVD layers from the outset than to create them later and then having to rework the applications you have already created. QVDs are extremely efficient; as soon as you have the requirement to read the same source data twice or more, one extraction to QVD and then reading (and if necessary, manipulating) a QVD multiple times is generally faster than reading the database table several times into each application that requires it.
Consider these scenarios: