It's important to draw a clear line between your own ideas and those of others. Citing and documenting your sources draws that line. It also describes the contextual framework, or "context", in which you wish your ideas, arguments and observations to be viewed; the larger conversation in which it should be placed.
When there are multiple sources and varying viewpoints in a discourse community's conversation, the contextual frame assigns intellectual and academic responsibility to the contributing authors. This is particularly important when forming an opposing argument or attempting to disprove the conclusions of someone else.
Proper documentation separates the various points-of-view in the source material and helps the reader see the path you followed in developing arguments that support your conclusions.