In addition to greater realism I find that weathering mutes the livery variations between Dapol and Grafar locos and adds heft to the model.

Light weathering to depict well-maintained locos that have been a while out of the shops (eg the MN and Warship) is certainly a good default choice but I chose the heavy weathering on the two BR Standards to add variation to my fleet and to evoke the run-down condition of many steamers in the 60s era.