They can use it as an additional energy source, especially when temperature is low. Be careful with this approach when using a typical layer feed as such diets are formulated to provide as little protein as practical when the feed is the only nutritional source without compromising egg production. The problem you can run into is that consuming too much scratch grains can dilute the intake of nutrients the scratch grains are low in (i.e. protein, vitamins, minerals).
I do not mix it with feed or give free choice, because chickens will eat the scratch grains before the layers feed. They should eat 90% feed to get the vitamins and minerals added to feed. They don't add supplements to scratch. I give my 5 hens 3/4 cup of 50/50 mix of scratch and layers feed twice a day scattered on the ground. GC