Fabric saves weight. It also enables fairly straightforward removal to inspect the underlying structure.
Although Guppy often supplies rather long-winded answers, this time...he is absolutely correct.
How do I (personally) know?
My Dad was engineering project manager at Douglas...(deputy manager, on the DC4), manager engineering on the DC6 and DC7.
In addition, fabric covering often allowed for a more robust assembly.
Even the Stratocruiser (a quite fast airplanre, for its time) employed fabic, in several locations.