The use of transitional words and phrases helps the readers better understand that which they are reading. These are words and phrases which bind together the meanings of sentences, making a whole out of the sum of the parts. When the trout are not rising, it is advisable to fish with a wet fly.
Cycling is good exercise, but jogging is better.
He is unscrupulous. Most readers would choose the second paragraph, primarily because it sounds more complete than the first. The only differences between the two, however, are the transitional words and phrases that help to bind together the meaning of the sentences. An essay is nothing more than a series of related paragraphs, and a paragraph is a series of related sentences. To help this relationship along, good writers use transitional words and phrases in their writings.