When you think about "dramatic significance" you need to ask yourself what purpose a character or characters serve in the play. The answer could be that the characters help to advance the plot, illuminate a theme, serve as a foil to the main character or some other character, or something else. The characters listed above all serve to illustrate some aspect of the theme of loyalty. In the play, loyalty is at the heart of the conflict: Claudius is disloyal to this brother and his kingdom when he kills King Hamlet and married his former sister-in-law. Hamlet's goal to restore a sense of decency to the throne of Denmark by avenging his father's death.
When Rosencrantz and Guildenstern arrive at Elsinore, it is NOT out of a sense of loyalty or concern for their friend Hamlet, but because they were summoned by King Claudius and are hoping for "a king's remembrance," a reward, for essentially spying on Hamlet and reporting back to the King and Queen about what is really going on with Hamlet. Hamlet is almost immediately suspicious of their motives, especially after they don't directly answer the question of whether they were sent for or not. Hamlet puts up his guard around them, and is sadly left in a more isolated state than before. Hamlet has only one true friend, or confidante, in the play, and that is Horatio. Horatio is a friend from school, but one who has been there fore Hamlet since the death of King Hamlet, not a late-comer. He loyally tells Hamlet about the ghost's appearance. He loyally swears not to reveal Hamlet's secrets about the ghost or his act of madness. He loyally agrees to help Hamlet watch Claudius during the performance of the play within a play. He loyally delivers the notes from Hamlet and keeps his exact presence back in Denmark a secret until the time is right. He loyally supports and congratualates Hamlet for the way he handled Rosencrantz and Guildenstern on the boat to England. He loyally cautions Hamlet about getting into the fencing match with Laertes, and finally, he nobly agrees to live on at the end to explain to the "yet unknowing world" why everything that happened, happened. Shakespeare is clearly illustrating the value of loyalty and the importance of frienship.