The reasons for European annexation of Cameroon were generally the same as the reasons for the Scramble for Africa in general. Some major reasons:
1. Resources. The African continent had many resources, especially mineral and agricultural, that were not available to the various European countries that participated in the Scramble.
2. Markets. It was hoped that having colonies would give each "mother country" a captive market for its goods -- one where only it could sell. This would help the European country's economy.
3. Strategic positioning. In these days, warships needed to be refueled relatively often and there were, of course, no airplanes, missiles, etc. So the only way for a country to be able to use military force in faraway regions was to have bases around the globe. Having colonies in Africa allowed this.
4. (And this may be the most important.) Prestige. If one country has a lot of colonies, then any country that doesn't have them looks weak (or at least weaker than those that did have them). As the previous answer indicates, this was a major reason for German annexation of Cameroon. Historians believe that this psychological need to keep up with the other countries was a huge factor in causing the Scramble.
As far as impacts go the colonization that followed the Scramble:
1. Hurt the colonies' economies. That's because their resources were being used to help the European countries rather than themselves.
2. Made it harder for them to do well once they became independent. One reason for this is that their economies were set up only for the benefit of the Europeans and this made them poorer than they might otherwise be. Second, the Europeans weren't interested in educating the Africans or in setting up democratic institutions. This is part of the reason why most African countries have had a hard time becoming stable and democratic.