These special wines, the stickies, have unique, highly concentrated flavors which are derived from the particular concentration process the grapes underwent and from the fermentation of the musts of very high sugar content.
Sweet wines have a very long history.
The wines of the ancients in the Near East, Egypt, Greece, were almost certainly made sweet. Greek low trained vines and the hot Mediterranean climate were able to yield grapes high in sugar content. The Romans initially grew vines up trees which allowed only low maturity and low sugar content. They concentrated grape juice by boiling. They also encourage oxidation of their wines to enhance their stability.