There were three major arguments in favor of imperialism. First, there was an economic argument that said that an empire would give the US sources of raw materials and new markets in which to sell US goods. Second, there was Alfred Thayer Mahan's argument about the importance of sea power. Americans wanted an empire to allow them to project naval force around the globe. This would allow the US to be a major military power. Finally, there was the idea that the US should "take up the white man's burden" and go out and civilize people in places like the Philippines.
Against this, there were a few arguments. One was that an empire was inconsistent with American values. Our country was built on freedom and we should not take the freedom of other countries. A much more racist argument held that bringing non-white people (like those of the Philippines) into our country would harm us. It would bring a lot of inferior people into the country, creating a burden on us.
These were the major arguments for and against imperialism.