Because the Industrial Revolution increased the production capacity of Western states astronomically, there was an enormous hunger for raw materials to satisfy demands. Thus, Western powers sought colonies where raw materials were abundant and where they could be appropriated at little to no cost. Additionally, colonies gave the Western powers a ready-made market for their goods, as the colonized people were left with little to no legal recourse to produce their own finished products. Technological advances in Western arms and transportation commonly made indigenous resistance to imperial incursions futile and short-lived, as Westerners had far superior weapons, ammunition, strategy and tactics.
There's 3 reasons, because it assured the Western economic and military ascendancy, it demanded both suppliers of raw materials and retail markets, and it provided ransportation and logical supports for conquest.
