The economic factors were desires to find new markets for trade. By extending colonial power throughout the world, the US would have new trading partners and markets. In addition, the US would be closer to new markets; when the US became a colonial power in the Philippines, it opened up trade with East Asia.
Politically, imperialism was able spreading nationalism/patriotism. It would be a point of pride to maintain colonies globally. It also makes sense from a military standpoint to have colonies in different regions of the world in case a military operation needs to be launched; this is the case with Guam, which was an American