Tasks describe the units of work you are doing for your accounts to help them achieve desired objectives.
Tasks also help inform the rest of the team on the work happening on the account. The Associate with users field helps you capture the account user for who the task is being performed. Documenting it helps in reporting tasks being performed for a certain user.
When the SuccessPlay with a user segment is created tasks that get created on accounts also have the user reference populated in this field so the task assignee gets the context of who the task is being performed for.