The practice of having a curtain separate between the sifrei Torah and the people is a well-accepted one. Tosafos (M'gila 26b) discusses whether the practice is to have the curtain inside the box housing the sifrei Torah or outside of it.* Although it is not discussed in these terms by that G'mara, the basis for having such a curtain (which you called a paroches) would seem to be the imitation of the mishkan, which had a curtain separating the zones of increasing sanctity - including the separation of the aron from the people.
I am not aware of grounds for an obligation to hang a curtain under normal circumstances, although T'rumas Hadeshen #68 implicates the need to maintain the Torah's personified modesty in his forbidding the temporary removal of such a curtain once it is already in place.
Rav Ovadya Yosef (in Y'chave Da'as 6:9) brings all of the above to bear in a response to the question of abandoning the curtain once a shul has come into possession of a very aesthetically pleasing aron. He strongly recommends against doing so, quoting Sha'arei T'shuva, who cites Zera Emes as a prooftext for the fact that having a curtain is a universal minhag. Incidentally, he specifies that the universal minhag is for the curtain to hang in front of the aron, not inside it.
Another reason for such a curtain could be the general principle of beautification of objects used in service of God.
*The matter under discussion is whether foreign objects are allowed to occupy the same space as sifrei Torah.