In The Tempest, what is the overall impact of the masque?

The masque highlights one of the themes of the play, that it is important to keep oaths.

The Tempest is a play about honor and loyalty.  Prospero tries to lead Ferdinand to fall in love with his daughter, but then he also tries to impress upon him the fact that an oath of love is a serious thing. 

Earlier, Prospero targeted Antonio and Alonso as oath-breakers for the role they played in stealing his kingdom from him and sending him away.  Ariel freezes them and admonishes them.

You are three men of sin, whom Destiny,
That hath to instrument this lower world
And what is in't, the never-surfeited sea
Hath caused to belch up you; and on this island
Where man doth not inhabit; you 'mongst men
Being most unfit to live. (Act 3, Scene 3)

The point is that they treated Prospero badly, and he wants to get revenge on them.  He will get this revenge by using magic to control them.  He takes their faculties from them, and cements them in place.  Then he has them led to him.

In the meantime, Prospero is allowing his daughter to marry the king’s son, despite the king’s role in his banishment.  He wants to impress upon Ferdinand the value of his daughter’s hand and the marriage promise in general.

Look thou be true; do not give dalliance
Too much the rein: the strongest oaths are straw
To the fire i' the blood: be more abstemious,
Or else, good night your vow! (Act 4, Scene 1)

During the masque, the two lovers hear from the goddesses Iris, Ceres, and Juno.  They demonstrate Prospero’s magic because he calls forth their images to bless the couple.  The masque promotes the element of magic and mystery in a less dreadful way than we have seen it before.  We learn that magic can be used to entertain and bless as well as for violence.

Prospero obviously wants Ferdinand to treat his daughter well.  He reminded him to refrain from physical intimacy before the actual wedding, and now he is trying to impress upon him the significance of the bond he is entering into and what it means to Prospero.  It should send a clear message to Ferdinand that his new father-in-law is quite powerful!