Fruit was a plant invention to tempt animals into seed dispersal and provide nourishment to herbivores without literally getting eaten alive – in other words, a diversionary stratagem.
The common take is that fruit is a meaty sugar with seeds. To a botanist, fruit comprises the ripened ovaries of plants, including the seed within. That definition – an ovary with seed(s) – is insufficient, as the definition also encompasses various nuts, grains, and vegetables.
A workable definition of fruit is a sweet-tasting gift by a flowering plant in a gambit to disseminate its seeds. By that characterization, there are ~2,000 distinct fruits in the world.
From an evolutionary perspective our sweet tooth descended from eating fruit. Most of the carbohydrates in hominids’ diets were found in fruits.
Fruit is readily accessible energy, typically with other wholesome ingredients tucked in. The fructose content of fruit is surprisingly small compared to its satisfying sweetness.
Starchy carbohydrates require greater digestion for equivalent energy. Whereas fruit sugar is an immediate energy rush, complex carbohydrates give up their sugars at a more measured pace.
22 mya, the African canopy was a year-round rainforest. Apes survived on the fruit from trees.
5 million years later, global cooling curtailed the fruit output, bringing a seasonality that spelled starvation. Hominids genetically adapted by becoming highly efficient fructose processors. Even small amounts in excess were stored as fat; a huge survival advantage for sweets that cannot be had year-round. Humans descended with the proclivity to tuck surfeit sugar away.
A brief survey of select fruits follows. (The fruits described were chosen based upon something interesting to convey. Practically all edible fruits are nutritious.)