Holding a button down constantly can get painful. If you’ve got to hold down a button pretty much throughout the entire game, say to run or crouch, that can get painful fast. Having a toggle on/off button option makes life a lot easier.
cyusha, via Reddit

For many hold vs toggle is preference. For disabled gamers is just vital. A game without toggle to aim is impossible for me.
Anthony Martins, via Twitter

Can u thank the person who decided to put auto sprint in Titanfall? Being a disabled gamer this makes the game much better!
Will Wright, via Twitter

Holding requires much greater motor ability than pressing, particularly on buttons that require more strength to operate, such as L3/R3 (pushing in the sticks).

While some degree of holding may be unavoidable, for example holding a trigger to accelerate or fire an automatic weapon, there are often others, that could instead be activated by a simple toggle, such as using a scope or looking in your rear view mirror, or even happen automatically, such as sprinting.

Playing using a toggle is marginally slower than holding/releasing, and automatic offers less control, so ideally offer a setting for players to choose which option they would prefer to play with.

Best practice example: Battlefield 4
Best practice example: The Long Dark
Best practice example: Uncharted 4
Best practice example: Titanfall
Best practice example: Counterstrike: Global Offensive