What could be the military advantages be for occupying an island

You are surrounded by water so it's harder to reach/ troops can't get to the island on foot, strong navy since you are surrounded by water, more isolation means less susceptible to attacks and raids, etc.

The  pan-african movement  was an "emotional, cultural, psychological and ideological  movement  that began among the  african  dispora in the western hemisphere, for a closer  purpose, so that  african  people could feel secure, attain political, economic as well as psychological power visa vis other races or world regions" .

The goods the europeans wanted the most were gold, jewels, silk, perfumes, and especially spices.