I am a physical education/kinesiology student, and in my human development and biomechanics courses (and a course I took in anthropology) point out that the pinky is useful in tasks requiring dexterity, but the great advantage of having a pinky finger is that it greatly increases the strength with which we can grip. (Try grasping something really tightly without using a pinky). When something is grasped between the palm and the fingers, it is called the power grip. In this position, much force can be applied.
Presumably, this characteristic was selected for during our evolution, as pentadactylism is an primitive trait in placental mammals, and an evolutionary trend in all primates. Strong grip strength is important for arboreal living, which all primates originally did. Tree-dwelling animals would not be able to climb vertically or cling from branches, let alone brachiate, without a very powerful grasp. Primates have evolved a deree of prehension of the hand and foot (except humans) wherby they are able to grasp; a monkey walking along a branch grasps that branch.
The presence of five digits in primates is called a primitive trait - it is primitive since it was originally present in all placental mammals. In many modern placental mammals, this feature has disappeared. Using the example of a horse, it is suited to running on hard ground at high-speed. This results in jarring and does not require forelimb flexibility. Therefore many primitive skeletal elements were selected against and have been lost in the horse, including the clavicle, decreased shoulder flexibility, the two bones of the lower arm have been fused, and the five fingers have been reduced to one, the hoof. Therefore, horses have lost those primitive traits. Horses have a highly specialized locomotor pattern, whereas primates are relatively generalized, we can carry out many forms of locomotion (quadrupedal, bipedal, and several variation thereof).
Also, on the issue of a base-10 numbering system due to having 10 digits - this is not necessarily so. Although the great majority of human societies use a base-10 system for this reason, not all do. In an anthropology class, the professor said that there was a group of people (can’t remember the name now) who used a base-8 system because there are eight spaces between our ten digits.