High value polarised capacitors typically do not have ideal characteristics at high frequencies (e.g. significant inductance), so it's fairly common to add a low value capacitor in parallel in situations where you need to worry about stability at high frequencies, as is the case with 78xx regulator ICs such as this.
One important point to remember about parallel connected capacitor circuits, the total capacitance ( CT ) of any two or more capacitors connected together in parallel will always be GREATER than the value of the largest capacitor in the group as we are adding together values.
We can also define the total capacitance of the parallel circuit from the total stored coulomb charge using the Q = CV equation for charge on a capacitors plates. The total charge QT stored on all the plates equals the sum of the individual stored charges on each capacitor therefore,
A couple reasons come to mind. Lower ESR. The effective ESR of the capacitors follows the parallel resistor rule. For example, if one capacitor's ESR is 1 Ohm, putting ten in parallel makes the effective ESR of the capacitor bank ten times smaller. This is especially helpful if you expect a high ripple current on the capacitors. Cost saving.
When large current peaks are drawn the capacitor supplied surge energy helps the regulator not sag in output. The white and black bars on the capacitor symbol show that it is a "polar " capacitor - it only works with + and - on the selected ends. Such capacitors are usually "electrolytic capacitors".
When capacitors are connected together in parallel the total or equivalent capacitance, CT in the circuit is equal to the sum of all the individual capacitors added together. This is because the top plate of capacitor, C1 is connected to the top plate of C2 which is connected to the top plate of C3 and so on.