you can imagine that when u try to pull large current into your chip, in transient, if u do not have the cap at Vdd, the supply current has to source all the way back from the power source (assuming no other cap is anyway near on Vdd), u cause havoc. the inductance on the power bus is going to glitch ur supply badly.
a decoupling cap should be installed as a general practice. if u want to filter the supply ac noise, u need a special filter to do that at the front stage, mentioned by the last poster.