You must remember that Seneca and Burrus are significant in the study of Agrippina the Younger for many reasons.
Firstly, Seneca was a proficient and well known and admired scholar in Ancient Rome. He actually tutored Nero. It is therefore considered by many historians that it was largely through Seneca that Agrippina was able to achieve the dominance over Nero that she was believed to have had.
Also, Agrippina relied upon Seneca and Burrus largely as confidante's and utilised their aid in securing Nero's rise to prominence, her marriage to uncle Claudius, and Nero's marriage to Octavia.
Thus their role is significant. Think about the nature and dynamics of the relationship between Agrippina and these two men and you should be able to understand their significance. Seneca and Burrus were largely in the service of Agrippina. It is interesting to note that after the death of Gaius, and the subsequent accession of Claudius, Agrippina even had Seneca and Burrus recalled from exile and placed back into her service (that also says something of the importance of the men to Agrippina) and as I said before, she inisisted that Nero recieve Seneca's tuition (another interesting point).
So I think that's what you should look at. If you look at the relationship between Seneca, Burrus, and Agrippina and the interactions between them at different stages, you will be able to make a strong argument regarding the importance of Agrippina's relationship with Seneca and Burrus.
Hope that helps!