Question

Why is yeast used to make wine and beer

Answer 1

We all know that yeast is used in process of fermentation. 
yeast converts the sugars of grapes into alcohol and carbon dioxide. This time, this fermentation is typical fermentation called alchoholic fermentation.

Hope it helped. ^^

Answer 2

Alcohol is a byproduct of yeast and as it consumes the sugar in wine and release CO₂ and alcohol.
The different strains of yeast for alcohol have their specific features to adapt to certain substrates (fruit, grapes, grain, beet, malt), enhance particular flavours in the finished product, adapt to certain temperatures, resist high or low temperatures, etc.