For more comfort
Just as we make our bed before going to bed and pat our pillows to make them softer, most dogs scratch their bedding to gain comfort. In doing so, they manage to find a sleeping configuration more to their liking in order to take a better nap or prepare for a good night.
Similarly, if their bedding is covered with a plaid or blanket and the dogs are a little cold, they can inflate their blanket by scratching and thus create a real little pocket of warmth, a cozy little nest much more comfortable to start a nap!
But, the reason our dogs go through a whole bedtime ritual - scratching, turning and stomping on their bedding - which sometimes seems a tad over the top to their owners, might just be fine beyond the simple search for comfort
Instinctive behavior
If a dog scratches its bedding before sleeping, it also seems that it is because certain ancestral habits inherited from its ancestor the wolf are still well anchored, even after thousands of years.
Before the domestication of the dog, wild canines dug holes to protect themselves from extreme heat and cold, as well as other predators. In the wild, circling and scraping the ground would have had the effect of flattening tall grass, both to form a comfortable surface, to conceal the animal's position while sleeping from other predators and to chase away snakes, insects and other undesirables from its bed.
This behavior would have even played such an important role in helping wild dogs and wolves to survive that it would have remained anchored in our "modern" domestic dogs who continue to adopt this instinctive behavior today even though this is no longer necessary.
A way to mark your place
Dogs have glands on the underside of their paws and between their fingers that secrete pheromones. By scratching their bedding, dogs could thus deposit their own scent and thereby mark their sleeping place as theirs, thus signifying it to other members of their social group.
You can also observe this little carousel every time you wash your dog's bedding or your sheets, if your dog is allowed to climb on your bed. Bed scraping is then a way of redepositing its smell on freshly washed sheets or blankets.