Within my own experience with Gantt Charts in industry, as others have said, it is a tool to help keep a project on track. It outlines the necessary steps that need to be taken, and the sequences they need to be done in so that a project can be completed in entirety and on time. One person made a point however that it documents what should be happening. As a project actually unfolds in reality, problems and events crop up that are not possible to account for, delaying the project timeline. For example, a project I was involved with was set to move into production at a certain date, but had components within it that overlapped with other projects. Since the product could not move to production until the overlapping project was closed, it delayed our timelines significantly (by months).
While errors and events happen and we cannot always predict them, the most important thing is reacting to these situations: What plan will you create in order to get back on track? What actions will help to achieve the best possible outcome, given the situation? In this case, we worked with those in the overlapping project to speed up closure, and in the mean time prepared all of our deliverables in parallel so that once the project closed, we could sail through our processes.