Hi, I used to do contract work with USAID in Ghana!

Since as far back as the Bush Jr admin, West Africa has been a focus of US foreign policy. Basically, West African countries want to develop a manufacturing sector, but lack the capital to do so. They have a hugely well educated population, but not nearly enough jobs. Meanwhile, the US wants to stop relying on China for things like textiles (China is geopolitically "complicated", labor costs are rising there, and the physical distance to ship from East Asia is much much longer than the distance to West Africa), and has tons of capital at its disposal. Thus, the two have found a common interest to jointly pursue. The Bush Jr administration made West Africa a special free trade zone, eliminating tariffs and encouraged US companies to invest in manufacturing there.

In Ghana for example, the money that USAID invests goes to many things: it helps to subsidize economic initiatives by the Ghanaian government to build infrastructure (roads, ports), manufacturing (textiles), upgrading agriculture (Ghana produces a huge amount of the world's chocolate!), and modernizing its medical sector (out in the countryside it was basically non-existent when I got there). I would imagine that this new investment of money will go towards bolstering some combination of those things.

As to what's in it for the US, it is trying to build good will and soft power in Africa in a geopolitical chess match with China, who is trying to do the same thing. The people of West Africa, as well as other parts of Africa, generally (but not always!) benefit from this competition, as the two Great Powers compete to essentially buy the loyalties of African governments and people ($139Mm is a drop in the bucket in the US, but it goes a long long way in a place like Ghana). China has generally been more successful with this effort, but Ghana is a specific example of a country the US has built a strong relationship with.