If the only op amp available is the dual (perhaps they have thousands stocked and for 1 product getting in a new part is more costly than using the dual device), then it is important to ground one input for EMC purposes as stated above.
This requires that it is added to the schematic.
Additionally, it makes it so that the schematic reader is aware that this gate of the IC is not used, that the unused pins shown have nothing on them and what they do.
Its an improvement on the old ways of not showing them and good practise.
What is bad practise is that in the schematic there are several junction points with 4 connections on them, the max should only be 3.