What is the purpose of the Hecuba speech in the play Hamlet?

This speech in Hamlet is spoken, of
course, by Hamlet after the Player King has delivered his moving monologue about
Hecuba.  Thisis an ancient tale, and, while it'sa tragic story, the characters and
events of this story are nothing personal to the actor.  Still, the Player King is moved
to tears as he tells the story.  Hamlet's speech which follows is generally known as the
"rogue and peasant slave" soliloquy. In it, Hamlet has two key themes.  First, he
berates himself for his comparative lack of emotion even for a just and personal cause
and is amazed at the actor's ability to create such emotion for something totally
disconnected from his life.  He says:

readability="18">

O, what a rogue and peasant slave am I!

Is it not monstrous that this player here,
But in a fiction, in a
dream of passion,
Could force his soul so to his own conceit
That
from her working all his visage wann'd,
Tears in his eyes, distraction in's
aspect,
A broken voice, and his whole function suiting
With forms
to his conceit? and all for nothing!
For Hecuba!
What's Hecuba to
him, or he to Hecuba,
That he should weep for
her?

Next, Hamlet notes that
if this actor had half the reason, the "motive and cue for passion," that he did, he
would be a rather wild man on stage, confounding all with his emotions and actions. 
Yet, Hamlet says, he "can say nothing"--not even against a king who has usurped the
throne by killing his own brother. He says he deserves all insults of word and deed for
this lack of resolve and passion.  He continues his diatribe against
himself: 

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Why, what an ass am I! This is most brave,

That I, the son of a dear father murder'd,
Prompted to my revenge
by heaven and hell,
Must, like a whore, unpack my heart with words,

And fall a-cursing, like a very drab,
A
scullion!

Then comes the
second portion of this soliloquy in which Hamlet makes a plan.  "About, my brain!" he
says, and then he determines to reaffirm the King's guilt by enacting a play within a
play (within a play, actually)--inserting a few lines into the play in order to catch
the King off guard and ensure himself of Claudius's
guilt.   

I'll
have these players
Play something like the murder of my father

Before mine uncle: I'll observe his looks;
I'll tent him to the
quick: if he but blench,
I know my
course. 

In the end, Hamlet
has--once again--made a plan to determine once and for
all:

...the
play 's the thing
Wherein I'll catch the conscience of the
king.