Tight scheduling feels efficient until one delay collapses the whole day. Experienced travelers build in buffer on purpose, treating it as insurance against the things they cannot control: traffic, security, and the plane that boards late.
Anchor buffer to risk. Give yourself more room before anything you cannot miss, such as a keynote or a one-way flight, and less before flexible items. A useful rule is to plan for the second-worst case, not the average, since the average is not what ruins a trip.
Use recovered time well. When things go smoothly, the buffer becomes a quiet window to prep, answer email, or simply breathe before the next thing. That margin is what separates a frantic trip from a composed one.