In addition to the reasons mentioned such as resource allocation, performance optimization and retention, there are also aspects of data curation that are to be considered here.

The bronze layer is often very close to the source that enables replay-ability as well as a point for debugging when upstream systems aren't accesible.  The silver layer enables deduplication and curation per enterprise needs, the base copy is still available in bronze for access as required.

The gold layer enables data blendin, look-up and enrichment of datasets for various use cases