Linking words or transitional words are those words or small phrases used to show the relationship between paragraphs or sections of a text or speech. Typically, they are used to make you understand the concept during the progression. Linking words or transitional words help the readers or listeners to progress from one idea (expressed by the author or narrator) to the next idea. Thus, they help to build up coherent relationships within the text.
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.