Assist modes are optional settings to compensate for lack of speed or precision, allowing gamers with difficulties in these areas to take part on a level playing field. They also have strong benefits for people who are simply less proficient at the game, allowing them to still experience the same progression and enjoyment.
They can be partial, for example requiring less accuracy for aiming, or full, for example carrying out all braking and gear changes completely automatically.
An obvious area of concern is the impact that this has on competitive multi-player, giving players a way to gain an unfair competitive advantage, but this is easily addressed through matchmaking preferences.
Best practice example: Dirt 3