The reason that Great Pyrenees dogs have 6 toes is because the extra toe is known as a Dew Claw and is useful like a thumb is to humans.
The Dew Claw is very useful and essential to the Great Pyrenees.
The Great Pyrenees Dew Claws are very strong due to being attached by a bone and they act as a sort of thumb for the dog.
The dew claws help the Great Pyrenees dog grip the ground for turning, climbing, descending, and jumping.
So while it might seem like the 6 toes are a deformity of the dog they are not and are born that way for a reason and they are very useful.
Some people have the extra toe removed but really you should not have it removed as it's a very useful toe for the Great Pyrenees dog.