What is one reason for leaving some beer in the bottle when pouring?

Leaving some beer in the bottle when pouring is primarily done to prevent the overflow of yeast sediment. Many beers, especially those that are bottle-conditioned, have a layer of yeast and sediment that settles at the bottom of the bottle. When pouring, if too much sediment is disturbed, it can lead to a cloudy appearance and an unpleasant mouthfeel, as well as potentially create excess foam. By leaving some beer in the bottle, you ensure that the sediment remains undisturbed, providing a cleaner pour for the drinker.

This practice is particularly important with certain styles of beer, such as Belgian ales or other farmhouse style beers, where yeast is intentionally left in the bottle for natural carbonation. It's also a common technique among cicerones and beer aficionados who aim to present the beer in its optimal state, enhancing the overall drinking experience.