Why are a number of African countries planting thousands of trees?
Planting trees slows down the wind and prevents it from eroding the soil too much. As the climate changes, the wind, moisture and temperature patterns are changing in many African countries and in some cases, soil erosion is increasing.
Many African countries also depend on trees for fuel cooking fires and many areas are getting very scarce in terms of sticks for fuel. In fact, in some cases, people are actively cutting down live trees for fuel wood which makes the erosion problem worse. Ideally, you would have a thick enough forest that people could gather dead wood for fuel and leave live trees intact for erosion prevention.
Under the Kyoto Protocol countries could also plant trees to help soak CO2 and sell carbon credits for cash. Its not clear if under the new Paris climate agreement this carbon credit scheme would still be operational.