Wayne Sheffield describes virtual accounts and how SQL Server can make use of them:
SQL Server will use these groups in many places so that permissions are granted to the group, instead of the actual service account. This simplifies things greatly if you change the service account – SQL Server Configuration Manager will just change the member of this group instead of having to hunt down and change everywhere that it knows that permissions are needed for the service account. Using these groups instead of the service account will simplify your life also if you ever change the service account – all those specific permissions that you granted on local resources (paths, registry, etc.) would have to be changed. Using the group, it will still have the same permissions.
I consider virtual accounts—particularly when you stick to using the virtual account itself rather than a domain account—to be a really good security feature, as it prevents system administrators from getting lazy and using the same service account everywhere. This in turn blocks an attacker from using a pass-the-hash strategy to pivot from one SQL Server instance to another.