Boundary line

Today I met friends V & C for lunch in Worcester Park. I parked the car in a residential street and when walking to the restaurant I spotted this boundary marker. I've not seen one like this before. There must be a reason why it was necessary to place it at the front edge of someone's garden. Very confusing when their drive is in a different borough to their house. It came in very useful for the estate agent though who used it to secure a For Sale board.