They are like the road signs that tell the reader that you are about to give an example, a comparison or a contrast.
Transitions create links in a chain.
In the parts of your writing where there is a major shift in focus, you can use transitional paragraphs. A transitional paragraph is a summary of what's been discussed thus far along with an indication of the direction the essay will take next. The chart below lists some transition words or phrases. Houghton Mifflin Co., 1995.