Game cover has 2 uses.
1st is, as you point out, to provide cover for the birds as they are growing and also as adults.
2nd is also to provide a source of feed too. The birds will eat seed from any quinoa, linseed, phacelia, sunflowers, maize, any cereals, beans etc. Other animals will eat the cover too e.g. deer.
As well as the above game cover is also a good source of pollinating insects and bugs which are a good source of food for birds.