Video: It's because though English is a Germanic language (the grammar and core vocabulary comes from that), there are a lot of words that come from the Romance (Latin-based) languages too, which were leaked into English when French-speaking Normans ruled England. That explains why there are a lot of twin words that mean the same thing in the English language. Think about it! Folk and people. Want and desire. Buy and purchase. Understand and comprehend. Needs and requirements. Woods and forest. Gift and present. Thought and idea.
Other words that are similar but used for slightly different things in English (like cow and beef and pig and pork) tell an interesting history between the relationship of English speakers and French speakers too. Watch as Akira Orent explains in the video below.