What is Hamlet's purpose in in talking to Polonius about the sun's breeding maggots in a dead dog? And why is the company of players traveling about?

To save confusion and in the interest of accuracy, I wanted to explain that the conversation the response below refers to (starting  Hamlet's comparison of Polonius to a fishmonger) is found in in Act II, Scene 2, line 175, not in Act III.  At this point, Polonius comes upon Hamlet reading and the players are not even present.  It is in Act III, scene 2 that a conversation takes place between Hamlet, the players and Polonius. 

As for the question about maggots, "spontaneous generation" was a common (yet totally false) scientific theory going back to as early as the middle ages.  People believed that maggots were already there, present in the flesh, and they sort of "sprouted" to life of their own accord once sunlight hit a decaying corpse.  Hamlet warns Polonuis that Ophelia needs to be closely looked after, since she doesn't really even have to do anything at all to get pregnant.  He asserts that all she has to do it walk around in public and it will spontaneously happen.  As with the maggots, Eve's original sin is already inherent at birth in all female flesh, so that babies will be generated by the simple act of walking around in public where men's eyes can touch or "kiss" their flesh.  This comparison illustrates that Hamlet has no faith in female virtue.  Instead, he points out the frail, fickle, easily corruptable nature of woman.  He thinks the only way to maintain female virginity is to forcibly lock them up (hence the reference to nunneries).  This ties in with one of the play's themes: mistrust (and perhaps hatred) of women.  Gertrude's actions so taint Hamlet's outlook that he can only see women as whores.  Desite all this, I think he still truly loves Ophelia, even while he enjoys mocking Polonius and hinting about his own desire to deflower her.      

The company of players are traveling around to make money, as would be common at that time.  Bands of traveling entertainers were something a village would anticipate and welcome--just think about how boring life in a castle estate would have been.  A permanent structure for performing drama or music before a crowd (like Greek ampitheatres or even Shakespeare's Globe) was such a rarity at this time that for the most part it just didn't exist.  In addition, these players mention that that they were pretty much forced to leave because of the new craze for child actors in the city.  No one is coming to see them anymore so they are forced to leave to make a living.  They're traveling for the same reasons musicians tour today:  for profit and self-promotion.    

Hope this isn't too simplistic...