On the sidelock, the hammers-sears-springs are all mounted on a plate that is seperate from the action/barrels. Generally a sidelock gun will cost much more than a boxlock because of the precision work involved for the inletting the locks into the stock.
In many of the British hunting shotguns/rifles sidelocks were a feature as the locks could be removed and serviced in the field.
Please note that some modern makers have put false side plates on boxlock guns. They look like sidelock guns but the plate is there for decoration and extra metal for engraving.