Parallelism is a grammatical structure that is used to emphazise an idea, repeat it and make it clear to the audience that it is important to remember. It reinforces memory and underlines the importance of certain information while not taking away from other parts of a text. In essence, paralel structure is a repetition of a grammatical form throughout a text which allows an audience to understand the significance of information and memorize it. In writing, as well as oral communication, paralellism is really important to make statements and ideas clear to the audience.
In the case of the excerpt, the author is transforming what could be a very flat and uninteresting piece of historical fact into almost a tale of the greatness of Rome and its empire throughout its peak and its fall. For example, the author says: "The frontiers of that extensive monarchy were guarded by ancient renown and disciplined valor." Aside from being parallelism because of the use of past tense, which makes it a similar structure but different ideas, the use of words like valor and discipline make the audience only think of the greatness of the Roman forces, even when they failed to achieve their goals of worldwide conquest. It also helps the audience to remember, even when the text needs to give the least desirable facts, that Rome was great through the good and the bad times and remained that way. This is the effect of paralellism.
