Some rather no-account fruits have been eaten for no better reason than that people were used to them. Eleagnus angustifolia , also known as Russian olive, is one.
If you look in a gardening book, you'll find this plant described as a small tree with olive-like fruits. You'd never guess the fruits were edible. In fact, if you tried them you still might say they're inedible.
However, Russian olive was one of the few fruits available to Mongolian nomads on the steppes of southern Siberia a thousand years ago, and the Mongols developed a taste for it. When they conquered China, they insisted on having good old Eleagnus fruits imported for the imperial table.
Then there's the sorb, also known as the sorb apple, which today is (probably) eaten more often than Eleagnus. It looks like a tiny, somewhat flattened apple, or perhaps a bloated rose hip.