There are other benefits though...
IE average productivity may improve.
Singapore will reportedly disconnect more than 100,000 government workers' computers from the Internet. As reported by The Straits Times, the measure has been adopted to stop leaks of information from the city-state's government to the wider world. Workers' own devices will be allowed to access the internet, provided they …
Won't this defeat the purpose and allow data leaks?
But presumably there is an audit trail for stuff passing through the mail server. It's a balancing act as for internet access for anywhere else: presumably staff have legitimate need to communicate with the outside and even at times forward official records or documents. However the fact everything is retained on the server in identifiable form acts as a deterrent to the malicious leaker.
> presumably staff have legitimate need to communicate with the outside
It's the wrong way to go about it though. If you allow staff to forward sensitive data to their personal accounts, then you have no idea where it went next. Whereas if you give staff access to their work accounts from home (via web access, VPN, or whatever) then you maintain the log of what was sent to whom. And if the employee does forward sensitive documents to their personal email account, you can discipline* them for it.
(*In Singapore, that means the cane. How I would love to take the cane to some of my co-workers.)