As a "part of speech" transition words are used to link words, phrases or sentences. They help the reader to progress from one idea (expressed by the author) to the next idea. Thus, they help to build up coherent relationships within the text and give the text a logical organization and structure.
English transition words are essential, since they not only connect ideas, but also can introduce a certain shift, contrast or opposition, emphasis or agreement, purpose, result or conclusion, etc. in the line of argument.
