to give credit to those people whose ideas/words you are using in your paper so that you don't plagiarize
to distinguish other people's ideas/words from your own ideas and words.
to make your argument stronger. Doing research on an issue strengthens your position, because it shows you have engaged with some of the other positions on your topic and incorporated them into your thinking.
to allow your readers to verify your claims and to get more information from the source materials.
Plagiarism is presenting another person’s words and/or ideas as your own words/ideas – either deliberately OR unintentionally.
To avoid plagiarism, give credit in your paper to the person whose words and/or ideas you have made use of. In other words, cite your sources. You must cite any source that contributed significantly to the ideas in your own paper, even if you don't quote directly from that source.