I think that it's simply a ready stockpile of inventory (if one can refer to such large bits and bobs thus). As there's space to store things, at least when the photo was taken, it would be easier to have ready fuselage and wing components delivered to the assembly site ahead of time, rather than lingering at the foreign manufacture factories in storage. Although AB are no-doubt an integrated enterprise with just in time deliveries of smaller components, the complexities of aircraft assembly can lead to hiccups that might derail the smooth running of the line at times. Better to have the frames there and not 'there'.