In the field of project management, things do go wrong and unexpected changes are always there. So, including a time buffer as a line item in the schedule is a way to account for unknown changes in the schedule. Also, I think this is a better way to avoid an extension of the project in order to protect the critical path from further delay instead of extending the project time in the schedule. Time restrictions help to use work time more efficiently. If the project proceeds faster than buffer times are consumed, everything is in the green, which is good for the team and the project. If buffer time is consumed more without real project progress, the project is critical, which is marked red.