I found the following article. It is not very accurate, and doesn't show much knowlege about the African tribes and customs, but I like the descriptions of the different kind of pieces to be found in tourist shops today.
Of course you can find erotic carvings in African Art, like the Sari Porno from Madagascar.
Also it is not true that most pieces usage can't be known. You can find plenty documentation,about most African Tribes.
Daily Observer Newspaper - African People & Culture: "African People & Culture
Written by Sanna Jawara Thursday, 13 April 2006
When a person views African art, several themes seem to come up over and over again.
These themes are representations of different things that are significant to African culture; and reveal the importance behind some of its most beautiful art.
The common themes are:
A woman and a child
A male with a weapon or animal
An outsider or 'stranger'
Couples are most commonly shown as freestanding figures of relatively the same size and stature. They may be representative of ancestors, a married couple, twins, or community founders. This is representative of the importance placed on two as one. Most art of this type was developed for shrines or for positions of ceremonial honour.
Sexual intimacy is rare in African carvings. This in that it is rare for men and women to display their affection publicly. The most common theme of the male and female couple is that of strength and honour; not love and intimacy.
The mother and child couple is often representative of mother earth and the people as her children. African women will generally have a very strong desire for children as well however. The strong desire that a woman has to bear children further shows the strong mother child relationship that is a vital part of African culture .
A male with a weapon or animal (commonly a horse) is commonly produced to show honour to departed ancestors. Animals are rarely sculpted for the purpose of showing the inward or outward beauty of the animal; but to give status to the person. Even today, many in Africa would consider the ownership of a horse to be of greater status than the ownership of an automobile. Showing a person with a horse would then be giving great honor to them. Sometimes people are shown with animals that are not really ridden; possibly even mythical. The purpose is to show the power given to one who rides such an animal; and the wealth that they must have.
As women achieve significance through their children, men will often be honoured in warfare. The one who goes into battle must have physical, emotional and spiritual energy to survive and to conquer. Thus the emphasis on weapons and the spoils of war in many African works.
A final common representation in African art is that of the stranger. In Africa, a stranger is someone from a different country or tribe. They would usually not be welcomed; and the more distorted the portrayal of the stranger, the greater the gap that is normally symbolized. Sometimes strangers; especially white foreigners; are given a form of respect based on their relatively great weaponry and other powers.
Unfortunately, much of African art comes with no explanation of the meaning intended to be symbolized when it was created. When the meaning can be discerned, the deeper understanding gained and fuller appreciation of the cultural heritage can be obtained in a more meaningful and memorable way.
Information and images kindly provided by Africa Imports