The scene in which we see Polonius's servant, Reynaldo, play alongside Polonius is certainly a very surprising scene. It is especially surprising because we never see Reynaldo again. However, the scene plays an important function. In a sense, Reynaldo is acting as dramatic foil to Polonius. A dramatic foils is a character that is used to portray the opposite character traits of another character. Therefore, Reynaldo serves as Polonius's dramatic foil, thereby revealing Polonius's darker, deceptive nature.
It is Polonius's idea to use Reynaldo as a spy to investigate his son's character and behavior in Paris. However, in this scene, Polonius actually reveals far more about his own character than about Laertes'. Polonius tells Reynaldo to first seek out any Danes living in Paris and converse with them about where they live and who they know, thereby gaining their trust as comrades. He then tells Reynaldo to ask about Laertes but by first making insinuations about Laertes' character, saying that Reynaldo doesn't know him very well, but has heard that he is wild and dishonorable, as we see in Polonius's lines:
But if't be he I mean, he's very wild,
Addicted so and so'; and there put on him
What forgeries you please. (II.i.19-21)
In other words, Polonius is telling Reynaldo to lie about Laertes' character, saying that he is wild, an addict, a gambler, etc. Polonius wants Reynaldo to paint Laertes as a wild, uncontrolled youth. His rationality is that in doing so Reynaldo will get their fellow Danes to speak the truth about Laertes' behavior. The fellow Danes will either affirm or deny Laertes' behavior thereby showing what's true, as we see in Polonius's line, "Your bait of falsehood takes this carp of truth" (70). In other words, Reynaldo's lies will catch the truth (eNotes).
The problem is that Polonius's plan has actually exposed a great deal about his own character rather than Laertes'. Polonius has shown himself to be a very cunning and deceitful man. In contrast, Reynaldo is an obedient and respectful servant who is interested in preserving Laertes' honor, as we see in his line, "My lord, that would dishonour him" (29). Since Reynaldo is shown to be the exact opposite of Polonius, thereby exposing Polonius's questionable character, we see that Reynaldo's purpose is to indeed serve as Polonius's dramatic foil.