When a player can choose to leave the main storyline and complete other tasks (ones not required by the game), these tasks are called side quests. The side quests provide the player with smaller goals to accomplish while waiting to accomplish the main goals.
Using Miyamoto's Pyramid in a different way, we can represent main quest as something the player has to complete. Side Quests are smaller less important missions that act like fillers when the main missions aren't being completed.
Give the player many smaller easier goals, so that the player isn't overwhelmed by the time it takes to achieve the main goal. It can also provide the player with an experience that the main missions aren't able to provide. It can also be used to give the player a secondary story to play through, closely resembling the story of life, which is many stories intertwined.
In The Elder Scrolls V : Skyrim the player has a choice to complete various missions, most of which don't even progress the story. The quests or side missions generally branch out from the main story and tell tales of their own, thus adding additional content for the player. These stories sometimes add to the narratives of the main quest.
The side quests should not contradict the goals of the main quests. The side quests should if at all amplify the main quest's importance.
Motivations :- Autonomy
Components :- Secondary Objectives