I had the same letter first time round. Maybe your employer didnt realise they should have sent something like this. Its just to confirm that the money is coming from the government and if for any reason the government turn down their application, they will not be able to pay the money to you.
It is very unlikely but its an insurance on their part just in case they are mistaken and do not qualify. If that is the case they will either have to put you on unpaid leave, with your agreement, or make you redundant, so lets hope its all fine.