Smoothing using Capacitors in power supply unit

(Q)Why do output capacitors always come in pairs in power supply unit ??? A big electrolytic and a small ceramic??? Why???

•  The answer to the large electrolytic is little bit tough. The simplest answer is that the caps are based on the design. Sometimes two caps and an inductor are used in a 'pi' filter(two big electrolytic and an inductor). Sometimes it is easier to use two caps to create one big one rather than the expense of buying one big cap. There is nothing wrong with using an inductor, but it is also based on the design. 

•  Second question regarding a big/small pairing .The typical Electrolytic/tantalum or ceramic cap combination is simple, the electrolytic can handle low frequencies, the tantalums/ceramics handle high frequencies, therefore you are improving decoupling. Electrolytic can also be a reserve energy source for current demand 'spikes'.

•    In some cases the electrolytic is for bulk storage and the ceramic is to get the ESR down. It's all for improved efficiency.