This is due to parasitical infestations dermatological infections and dog hair being a breeding ground for bacteria.
Dogs coat is matted.
Matted and knotted fur can be very painful for your dog and even more painful to try and brush out.
Ignoring matted fur in your dog and not maintaining a healthy coat can lead to problems with hygiene in turn posing significant health risks.
Sometimes the only way to assure that the dog has been brushed thoroughly is to go over the entire body with a metal comb.
Brushing the outer coat will make the dog look good but may not get at all the mats.
Problems with matted dogs.
That degree of matting is extreme but all dogs can get tangles and knots in their fur.