Lets talk about Matts
Matts are solid lumps of fur, so tangled that they cannot be combed or brushed out, they are as painful for your prince or princess as you getting your hair caught in a zip.
Being completely matted is like living in a straight-jacket that prevents them from moving, sitting or lying down without discomfort or pain. They can occur anywhere on the body, but most frequently in high friction areas:
Behind the ears and around the collar area
In the armpits, groin and under the tail
Between the pads and the toes
They can hide all sorts of nasty things like ticks, fleas, thorns and twigs, which would hurt your prince or princess every time they move. They can constrict the circulation and can get so tight they tear the skin.
You cannot clip through matts, only underneath them. Cutting them risks cutting through the skin. Brushing them out is very painful and contrary to the 2006 Animal Welfare Act. So in severe cases (more than 10% of your dog) a total clip-off is the only option.