I think the text is pretty self-explanatory.
This is for workers who undertake essential or emergency work in the UK, or are returning from conducting such work outside of the UK, where a department certifies that the work is necessary to facilitate essential government work or essential state business.
This exemption will only be granted on a case by case basis, in exceptional circumstances, where the work is sufficiently urgent and critical that undertaking a managed quarantine period is not possible.
The exemption would be granted
exceptionally, if there is something vital that the government requires you to do. The exemption is being given by the government so that it can get some specific work out of you. Which is why a government department needs to sign off on that and certify that you need to be given that exemption. It is not a blanket exemption.
I am not a lawyer or immigration advisor. My statements/comments do not constitute legal advice. E&OE. Please do not PM me for advice.