1. With the British Empire extended all over the world, the need for laborers was great. Thus, the slave trade in England begain around 1662 when John Hawkins, the first British slave trader captured 300 slaves in Sierra Leon. (The Dutch Jews are credited with being the first actual slave traders, however, in 1619.)
2. Added to this, the British in the colonies were not numerous enough nor naturally adapted to the sun (there was no sunblock lotion back then) and tremendous heat of the equatorial climates, so there was a need for indigenous people to work for them. In some areas, there were not enough native people, so others were brought in to work.
3. Finally, by enslaving people, there were profits to be made through these sales of the slaves as well as through the sale of the product that they reaped. (cheap labor)