Why is Fortinbras's presence important?Hamlet (act 5)

In Shakespeare's Hamlet, the appearance of Hamlet's foil, Fortinbras, a man of brevity, loyalty, and bravery, provides the impetus to Hamlet's final actions.  Like Hamlet, Fortinbras has had a father slain, but he immediately seeks revenge.  But, unlike Hamlet the procrastinator, he is, as Claudius brazenly calls him "an opportunist."  Horatio warns that Fortinbras (whose name suggests strength [fort=strong]) "is going to recover of us by strong hand" the lands that King Hamlet has taken from his father. And, like Hamlet he is the true heir to the throne of his country, the Prince of Norway as Hamlet is the Prince of Denmark.  The loyalty of Fortinbras impels him to avenge his father; however, it also causes him to faithfully obey his uncle's wishes not to bear arms against Denmark while Hamlet waivers between thoughts of revenge and despairing ideas of suicide.

Finally, in Act V, when Hamlet who waivers in his loyalty to his murdered father witnesses the valor of Fortinbras, he is in awe.  In his final soliloquy, Hamlet reflects,

A thought which, quartered, hath but one part wisdom

And ever three parts coward--I do not know

Why yet I live to say "This thing's to do,'

Sith I have cause, and will, and strength, and means,

To do't.  Examples gross as earth exhort me:

Witness this army of such mas and charge,--

Led by a delicate and tender prince,

Whose spirit, with divine ambition puffed,

Makes mouths at the invisible event,

Exposing what is mortal and unsure

To all that fortune, death, and danger dare,

Even for an eggshell.... (4.4.46-53)

Fortinbras provides Hamlet with the inspiration to act:

....How stand I then,

That have a father killed, a mother stained, (4.4.56-57)

Unconsciously fulfilling the desire of the Prince of Norway, the Prince of Denmark in his valor rids Denmark of its corrupt court.  As all of the court of Denmark are slain, fortuitously, Fortinbras enters.  And, ironically, the looming threat of this Prince of Norway's revenge is fulfilled, not as a threat, but as a reward to him who has "some rights of memory in this kingdom" (5.2.369). It is also ironic that Fortinbras's inaction of not attacking Denmark provides him reward, while Hamlet has suffered because of his inaction.   Nevertheless, in this reward to Fortinbras, that "delicate and tender prince," Hamlet acts with valor and achieves victory over the corrupt court of Denmark, receiving a soldier's burial by decree of Fortinbras., who declares,

The soldier's music and the rite of war

Speak loudly for him. (5.2.178-179)