Attendance is an important part of your Fan Club. The more fans attend your match, the bigger the Possession Bonus of your team. 
Also, make sure to visit the Stadium section and set the ticket prices right. A high ticket price might cause some supporters to miss your Home matches and decrease Stadium attendance.

The loyalty and dedication of your fans will fill out the Loyalty Bar, triggering one of the Surges - The Attendance increase! Once the Attendance Surge is active, your Possession bonus will be increased depending on the Surge level.