If the digital side could be trusted we'd just set it to send only mode and be sure it'll behave - in reality we don't trust it. The threat model where you use

If the digital side could be trusted we'd just set it to send only mode and be sure it'll behave - in reality we don't trust it. The threat model where you use a data diode presumes an adversary might totally compromise the sending side - the guarantee you're trying to add is that whatever malware they push down the line has no ability to exfiltrate data regardless of how compromised it is.