Technically they do not -- true Gantt charts as envisioned by Henri Gantt do not even show how the tasks are related (e.g. dependencies). Thanks to the Excel-like engine that MS and others use behind their Gantt-like chart tools, you can do things like:
- record actual hours and budget to show against estimated
- mark off completed tasks
- show amount of progress on in-progress tasks
- view how resources are utilized and if they are under/over allocated
- calculate earned value
- perform hundreds of other calculations and create custom reports