When using wine in reduction, a general rule of thumb is wine should be reduced by at least 50% before using it to create a sauce. This helps build intensity, complexity, and body for the sauce.
Reduction of wine is essential for good sauce making. Evaporation helps intensify the flavors in wine, reduces acidity and reduces alcohol. Reduction leaves you with the essence of wine, which can then be used to flavor sauces without thinning them out.
If you reduce a liquid containing herbs, such as garlic, it helps to draw out these flavors into the sauce.