Shout
Shouting is an ability intrinsic to every playable species and most monsters. It allows the shouter to call attention to themselves and is an option found under the t key.
A character’s shout normally has a loudness of 12, roughly equal to loudest melee attacks, and roughly half as loud as a scroll of noise. Keep in mind that in some branches, sounds such as shouting may travel more or less distance. More information can be found in the noise article.
Both players and monsters shout for the same reason: to awaken and draw forth sleeping or wandering monsters nearby. A player might do this to force an enemy to open a door or to draw a few monsters from a crowd at a time, though shouting from the wrong position may bring an army down on you. Monsters who can shout often do the moment they see you; if you're concerned about the possibility of reinforcements, you may wish to retreat from enemies that shout near unexplored areas, luring the shouter away from anything that may have heard its cry. Also, bear in mind that your attacks are often noisy in and of themselves.
Like most in-game tactics, the benefits of shouting are situational. It is rarely something that stealthy stabbing-focused characters will want to do, while worshipers of Ashenzari and characters with access to telepathy or the antennae mutation can often use it to great effect.