Front dewclaws help dogs grasp things using their forelegs, such as a toy, ball or food. Although dogs can hold on to things without a dewclaw, the dewclaw is a helpful addition for grasping.

More important, perhaps, dewclaws most likely help stabilize the carpus (ankle):

• For agility dogs, leaving the dewclaw in place helps them with tight turns and most likely prevents arthritis in the carpal joint as they age.
• The dewclaw actually does touch the ground in racing and coursing breeds when the dog makes tight turns.
• Rear dewclaws in climbing breeds such as the Saint Bernard and Great Pyrenees are thought to help them while climbing rough terrain.

Generally, however, when there is a dangly, floppy rear dewclaw, it serves no purpose. There is probably a benefit to dewclaw removal in these cases since they can get caught and become injured.

Front and Rear Dewclaws

• Front dewclaws: All dogs are born with a dewclaw on each front leg.
• Rear dewclaws: Some dogs are born with 1, 2 or even double dewclaws on the rear legs.

Rear dewclaws are considered an anomaly except in a few breeds where single or double well-developed rear dewclaws are part of the breed, such as the Briard or Great Pyrenees. These should not be removed.

SOURCE: https://www.petful.com/pet-health/dewclaw-removal-dogs/