The naive definition of backup is "copy all data again in one go." But if you copy a 1TB volume every hour, you accumulate 24TB a day, 720TB a month. That's not backup—it's a cost bomb. EBS Snapshot is clever because it abandons this naive definition and adopts an incremental model: "record only the blocks that changed since the last backup." The first snapshot is full-sized, but every subsequent one only stores changes. Even a 1TB volume, if only 10GB changes daily, the second snapshot uses only 10GB.
Every characteristic of EBS Snapshot derives from this single design decision