First, the Scrivener’s speech is actually a sonnet, in that it has 14 lines.
Although it doesn’t follow the rhyme scheme of the sonnets that Shakespeare used to write, form in literature – more often than not – always has a function.
So the use of the sonnet is meant to reflect the status of The Scrivener as a learned man; a man of letters – much like Shakespeare.
Second, the speech is right smack in the middle of the play – and it’s a 14-line scene on its own.
Why was it so important as a scene that it had to be left on its own? Why couldn’t it have been excised?
Shakespeare already had enough material in the play to show the purported misdeeds of Richard, along with the purported views of the citizens.
Why does this scene even have to exist?
Last but not least, if I may take the liberty of paraphrasing The Scrivener’s speech slightly, look at what we have (mentions of time shouldn’t be interpreted literally):
This is the indictment of the good [King Richard III];
Which in a set hand fairly is engross’d,
That it may be this day read over…
And mark how well the sequel hangs together:
Eleven hours I spent to write it over,
For yesternight…was it brought me;
The precedent was full as long a-doing:
And yet within these five hours lived [King Richard III],
Untainted, unexamined, free, at liberty
Here’s a good world the while! Why who’s so gross,
That seeth not this palpable device?
Yet who’s so blind, but says he sees it not?
Bad is the world; and all will come to nought,
When such bad dealings must be seen in thought.
Convinced? Or am I also using media and language to manipulate your mind?