Do your fish spend a lot of time at the surface sometimes gulping air? Do you have a bad smell coming from your pond? Then you most likely do not have enough circulation. If a large number of fish die in your pond then you may not have sufficient oxygen in your pond.
If you already have a waterfall or fountain it may not be adding enough circulation so you may want to consider a larger or additional pump. You may also want to consider adding an additional fountain or waterfall. It may also be necessary to remove plants and fish when these become excessive.
However by simply adding an air pump you can greatly increase the amount of aeration and circulation in your pond without going to the time and expense of adding additional features to your pond which may not have as effective results as an air pump.