The reason why Lockrings are used at every coupling is to make them as tight as possible for a good electrical and mechanical connection, especially when conduit is hovering from screwed rod etc. The reason why serrated washers (jaggy washers) are used is for a similar reason, they give an extremely good electrical connection, especially for metal clad boxes. The thinking is that the conduit is still classed as an earthing conductor if required and to keep good practice going, even if the modern standards are slipping!