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Dewclaws are essentially the thumbs and big toes of the dog world.

They are not directly equivalent to the human structures of course, but they are similar. Looking at the a dogs front foot, the toes that are in contact with the ground while standing correspond to our pinky, ring finger, middle finger, and index finger. The dewclaw is the "thumb". 

A dewclaw that is attached by bone to the dogs front foot has a definite purpose. When dogs run, their front feet often bend to the point where their dewclaws come in contact with the ground. At high speeds (especially when turning) or on slippery surfaces, these dewclaws provide extra traction and help stabilize the carpal (wrist) joint. Some dogs also use their dewclaws to help them climb large rocks, trees hold objects to better chew on them, or climb out of the water  in difficult  situations .

Because front dewclaws serve an important purpose, they should not be removed unless there is a very good reason to do so. In very rare cases, a dogs dewclaw may be severely injured ,or develop a disease

(e.g.,. cancerous tumor) and removal under those circumstances would certainly be in the dogs best interest. These problems occur so infrequently, however, that removing healthy front dewclaws to prevent them makes little sense.