One of the most powerful benefits from being there since the start is the complete confidence in ripping out and deleting obsolete code later. Even good developers new to the project are afraid to do this, and they should be since it's very risky without the full context.

The natural trajectory for a project is to keep adding features until it collapses from its own weight. Only the long tenure developer can fight this and revitalize a project by removing the useless excess.