This mechanic is fairly straightforward as it is used to give the player extra rewards that will benefit them further in the game or simply make them feel better about themselves. Sometimes bonuses can be received if the player has completed a hard task, other times it can happen no matter what so that the player enjoys the game more.
A good example of a game that uses this mechanic is Super Mario Bros. After completing a certain amount of stages the player will load into a bonus stage. This will be in the form of a stage that is filled with coins that the player can collect to increase their score.
Similarly, too before this is also done in Crash Bandicoot, however, the player is timed. The player runs through the course collect as many apples as possible but can not go back if they miss any. This benefits the player in the same way as the other example as it increased the player’s score.
If I were to use this mechanic in my own game I would make it so that it works alongside the combo mechanic. If the player is able to complete a combo or a shortcut that the game did not tell them about then they will receive a bonus award.