Scanning your travel pass or digital boarding will check whether your subscription and MOBIB card are still valid. Even if you have paid for a travel pass, the validity of the subscription must be verified with every ride.
By scanning/registering your boarding and transfers, De Lijn also knows how many people are on buses or trams at any given time. We use this anonymous data to more accurately map capacity on our vehicles.
If you don't scan or register, you risk a fine.