The output from an unregulated power supply unit or a DC voltage source from a wall adapter contains many ripples also known as noise. these ripples are inherent in the AC voltage signal and sometimes come from spike or electromagnetic interference and no amount of filtering will totally get rid of these noise.
The ripple is not good for many electronic devices because it affects the function of sensitive electronic components such as transistor and integrated circuits, the problem is worse for digital circuits. For normal analog application, when the load draw from the supply, the ripple is more severe results in reduce working voltage, therefore the equipment will not deliver as expected and probably damage it.
To secure a well filtered and well regulated DC voltage for devices to function as required, we have the fixed and variable voltage regulators; the fixed varieties include 78xx and 79xx for positive and negative regulators respectively. The adjustable types include LM317 and LM337 for positive and negative variable regulators respectively. Check the pin-outs for these components before insert them in circuit. Simply add any of them to a filtered rectifier circuit or AC wall adapter and you are set.
The voltage from the regulator is straight and devoid of damaging ripples. You need one for your next electronic project design and construction.