I assume that you are asking about the Simon Fraser after whom the Fraser River in British Columbia. If so, the purpose of Fraser's voyage was to establish trading posts and routes of travel through the area. He was also trying to take possession of the territory for Great Britain.
Simon Fraser was working for the North West Company, which was involved in the fur trade. The company wanted to establish trade with the natives of what is now British Columbia. It was for that reason that Fraser explored in that area. In the course of that exploration, he descended the river that now bears his name. His efforts helped to set up trading posts in the area and to establish Great Britain's claim to that territory.