Why do some beers have high alcohol levels and some have low ones? Is the alcohol content something that can be changed for any specific brew, and the brewer sets it at a specific level on purpose? Or is the alcohol level something that is achieved in combination with other factors in the brewing process (so for example, a particular taste will always coincide with a specific alcohol level)?
Beer is an alcoholic beverage produced by the saccharification of starch and fermentation of the resulting sugar. The starch and saccharification enzymes are often derived from malted cereal grains, most commonly malted barley and malted wheat.
It can be influinced and is often influenced during the brewing process, by adding sugar (which gets transformed into alcohol) at certain phases during the brewing process. Several beers first get brewn on a barrel and then after a few weeks put onto a bottle after which sugar may be added. Adding the sugar causes a seconds fermentation which will result in a higher alcohol level.
Often deeper tastes as experienced with for instance a La Trappe, Rocherfort 10 or Westvleteren are the effect of the higher alcohol level. While alcohol certainly plays a role in the beer's taste, spices and herbs are also important for the beer's taste body.