Saving a person they love or trying to fulfill the promise they weren't able to complete. When any character(s) in a book world have strong emotions and/or feels extremely unwilling about something, be it getting married, facing a stubborn toxic pursuer or their own or someone else's death, they are able to perceive the system plane and send their wish. If the wish is passed, they either give a task if they want to ask for assist or they become taskers if they want to do so themselves, at a price.
A task giver provides 20% soul power, be it less or more, to the tasker upon completion and 5% to the main system of the system plane. Upon task completion, the task giver goes off to reincarnation, if he is dead already as the task done is in a parallel plane, or reoccupy his own body, if he was alive at the time when tasker descended. The tasker can't forcibly occupy a body for their use if the task giver is alive at the time when the task was given. It's a different matter if the two reach an agreement and form a contract.
A tasker, who becomes one on self-initiative, provides 30% soul power to the main system, when registered, and assigned a temporary system. Depending on the time flow of their world in comparison to the system plane, it is decided whether this task is to be done first or later. And a tasker, who's recruited by a system, provides 15% soul power to main system and 10% to the system who recruited them when he's registered.
Initially, most taskers start from a novice, E-level, task and by the time they start doing B-level tasks, they become stable taskers. After becoming stable, many of these taskers also start showing their quirks. Some only accepting tasks who have same name as them or same appearance, or they apply to use their own body instead of the task giver, some are attached to particular occupations such as only being a CEO, a doctor, or a teacher. Some even like to go in an amnesiac state and only get their systems to tell them when they are at crucial points. Simply, you can find all kinds of taskers out there with all kinds of quirks.
Most of the tasks are performed in parallel planes as a large number of task givers are dead by the time they give a task. Before their soul enters the reincarnation cycle, they get rid of their obession through this way usually. They get to see a different possibility.
In any task, only 10% ooc (i.e. out of character) is allowed. Anymore ooc and the points would be deducted due to less satisfaction of the task giver. So, taskers often choose a task that's close to their own personality. But, there are also those taskers that test their acting skills being someone different than their own personality.