Crouching allows players to reduce their height, allowing them to dodge damaging enemies or projectiles, makes themselves less visible, or enter tight spaces. In some games, including many 2D platformers, players can't move while crouched, while in others, mainly shooters, they can move at a limited speed. While the mechanic existed for years in 2D shooters, platformers, and fighting games, it's a newer feature to 3D shooters. The first 3D game to include the mechanic was System Shock, which was generally known for the variety of player actions it afforded. In PC games it is common for crouching to be bound to the Ctrl or C key, while in console games players commonly crouch by clicking in one of the control sticks.
Most commonly found in console games, player characters will sometimes automatically crouch when they reach an enclosed space such as a vent and will stand automatically when they leave the space. In games with cover systems characters will also crouch down to hide behind smaller objects when the player utilises them as cover.
Crouching by Genre
In shooters specifically crouching may have an effect on gun combat, giving players greater accuracy with weapons or allowing them to focus their aiming reticle faster. This is done to both add depth and strategy to gameplay, as well as making games more realistic. In some shooters it is possible to "crouch jump" where crouching mid-jump allows players to reach higher places than jumping alone.
In stealth games crouching is often essential for players being able to move about the environment without being detected by enemies. Crouching reduces the visibility of the player character and where applicable, the amount of noise they make.