This is what Polonius has to say about that: “What was I about to say? By the mass, I was about to say something! Where did I leave?” In other words, not even Polonius really knows why he sends a spy. There's no special or good reason. Instead, we must look at what that shows us about him and the court: he spies on his children out of habit, and distrusts them automatically, assuming he knows best. This shows us that the court is a place of distrust, though not, ironically, where it needed to be. (Hamlet didn't suspect his uncle in time to save his father.)
In Act II, Scene i, Polonius's ostensible reason for sending Reynaldo to Paris is to take money and notes to Laertes. His real reason, however, appears to be that he wants Reynaldo to spy on Laertes and find out if he is gaming, drinking, fencing, swearing, or going to brothels. To put it simply, Polonius does not trust his children and keeps an ever-watchful eye on them. In Act I, Scene iii when Laertes is preparing to leave, Polonius is eager to lecture him in great detail about how he should behave abroad. His meddling ways are not limited to his son; Polonius also tells Ophelia to stay away from Hamlet so that her chastity is not in danger.
I have been very puzzled by this. Its interesting that he is quite happy for Laertes to be see to be 'one of the boys' with his excess, but he never gives Reynaldo a clear answer as to why he is being sent. In fact, he rather conveniently fumbles the answer by forgetting what he ahd just said - in fact, in the end Reynaldo doesn't press him further as to why and sort of leaves the vague reason as a bit of fatherly snooping. But if that was the case, then why not just send Reynaldo to spy on him secretly without giving impressions of bawdyness. In fact, Polonius does say at the end as an aside - 'well see how he is doing with his music'
No there is something much more sinister here or we wouldn't have it in the play. But what I don't know!!!! and how it comes into play later with Hamlet.