Tobacco was important in the Virginia colony because it was the basis of the colony's economy from the earliest days of that colony.
The Virginia colony had its beginnings in 1607 when the English landed at Jamestown. By 1612, John Rolfe had perfected a way to grow and to cure (prepare for smoking and other use) tobacco. Demand for tobacco in Europe was so great that tobacco became by far the most planted crop in Virginia.
Tobacco caused at least two important negative effects. First, it depleted the soil quickly and led to the English needing to expand and take more land. This helped lead to conflict with the Indians. Second, tobacco production needed a great deal of labor. This led to a great demand for slaves.
Tobacco, then, became the backbone of Virginia's economy and led, among other things, to the colony's dependence on slave labor.