because the bores become very shiny and lose the pattern and it is considered good practise to have a very light hone when fitting new rings to give a good seal and to keep oil on the cylinder walls and to stop metal on metal concerning the wall & rings
last thing you need after spending time rebuilding is it to start burning oil
the cylinders will look glazed and if that is the case it will need honing
you can either send it off and it will cost around 30 - 50 or just get a very fine wet n dry paper like 600 and give it quick once over in a cross pattern
i heard of people using a scotch pad and warm soapy water - the idea is to remove the shiny glaze off it so oil will stick on the cylinder walls
first though u need to ensure your cylinder walls are in good nick and there is no play in any of the bearings