Transition words provide useful information in sentences and paragraphs. In writing, transition words help the writer get from one idea to another; they supplement a piece of writing and provide a smoother flow to the writing style.

Relationships

Transition words often describe the relationships between ideas. In the classic sentence, "I think, therefore I am," "therefore" is a transition word that describes the relationship between the speaker's ability to think and his very existence.

Cause and Effect

Transition words can be useful in describing cause and effect relationships. For example, a writer might say, "I ate the sandwich, and consequently I am not hungry." The word "consequently" is a transition word that explains that the cause of this situation is eating the sandwich and the resulting effect, or consequence, is that the writer is no longer hungry.

Sequencing

Sometimes a transition word or group of transition words might be used to describe a sequence of events. So a writer might say, "First I went to the store, then I came back home." "First" and "then" are both transition words in this sentence, used to explain the order of events that happened in this writer's day.

Compare and Contrast

Transitional words can help a writer compare or contrast two ideas or things. For example, a writer might say, "Bicycle A was well-constructed; conversely, Bicycle B was poorly made." The word "conversely" shows that there is a discrepancy between the construction of the two bicycles.

Emphasis

Transition words can provide sentence enhancement by showing emphasis. Words and phrases such as "furthermore," "in addition" or "especially" work in this fashion.

Source:

University of Richmond: Transitional Words and Phrases