What is a common reason for leaving a small amount of beer in the bottle when pouring?

Leaving a small amount of beer in the bottle when pouring is often done to avoid serving yeast unless specifically requested. Many types of beers, particularly unfiltered or bottle-conditioned varieties, may have sediment at the bottom of the bottle. This sediment is composed of yeast and other particulates that can alter the flavor and mouthfeel of the beer if poured into the glass.

By leaving a small amount in the bottle, the pourer can ensure that the drinker enjoys the clear, more refined liquid first and can choose to mix in the sediment if they desire. This consideration enhances the drinking experience, as some consumers may prefer the clarity of the beer without the sediment.

The other choices explore valid considerations but do not directly align with the common practice of leaving beer in the bottle. Preventing spoilage pertains more to proper storage and serving techniques rather than the act of pouring. Reducing alcohol consumption doesn't factor into why one would leave beer in the bottle during a pour, nor is it generally a reason someone would finish a bottle. Ensuring the bottle is empty is contrary to the intent of leaving some beer behind in the first place.