In CFD a computer solves the discretized equations on a numerical grid which should be fine enough in order not to neglect important features of the flow in between. Beside of the real experiment and pure theory it has established itself as the third column of fluid dynamics. In contrast to the real experiment, the numerical experiment offers much more data. Pure theory has the disadvantage that it can only provide solutions of few simple problems. It shall not be concealed that by far not all situations of interest can be simulated accurately. For example, in most cases turbulent flows have to be modelled which involves simplifications.
A mocking interpretation of CFD is Colors For Dollars, since the visualisation is very colourful and the software in most cases expensive. Alas, this is only the opinion of the mockers! 🙂