Hi, If the composite  filling is rough food gets stucked on to the rough surface, they are not easily removed like natural tooth.over a period of time discoloration occurs sovbetter to have smooth suface fillings

A polished surface is self cleansable and doesn't harbour microorganisms as opposed to a rough surface. It is also comfortable to the patient. It is therefore desirable to smoothen the composite filling for better patient acceptance and a good prognosis of the filling.

Fillings have  minor irregularities(not visible through naked eyes) on its surface.So if it's not polished thn whatever food or liquid you consume will get entrapped in that minute spaces.It will not be cleaned by your normal brushing.In long run this vl change colour of your filling. It's always advisable to smooth surfaces   of fillings.