Dogs almost always have dewclaws on the inside of the front legs and occasionally also on the hind legs.
For certain dog breeds a dewclaw is considered a necessity, e.g. a Beauceron for sheep herding and for navigating snowy terrain. As such, there is some debate about whether a dewclaw helps dogs to gain traction when they run because in some dogs, the dewclaw makes contact when they are running and the nail on the dewclaw often wears down in the same way that the nails on their other toes do from contact with the ground. In many dogs the dewclaws never contact the ground. In this case, the dewclaw's nail never wears away, and it is often trimmed to maintain it at a safe length.
The dewclaws are not dead appendages. They can be used to lightly grip bones and other items that dogs hold with the paws.
Info. from: en.wikipedia.org/wiki/Dewclaw#cite_note-Penn-2