One major reason is parcel and luggage. I have myself observed this in MAS when train comes in especially 13352. There is lot of parcel which is loaded from MAS for this train. In PER this facility is not available. I also remember hearing the same from a rail official. Along with 13351, Raptisagar, Ahilyanagari and Korba-TVC are the only pass through trains which I think are touching Chennai