A sling bag fills the gap of needing a compact pack that can store multiple items without occupying your hands. The style of a sling bag, therefore, makes it a lot lighter than a backpack and considerably less stiff for your back and shoulders.
That said, it can’t replace a backpack that is able to fit 10x more items. But it can act as an add-on for, say, long-distance air travel and a weekend getaway in the countryside.