My grandfather cites Midrash Rabbah, which is also cited by Rashi, that explains this parsha was the one said at Hakehel because many points of Torah are within it.
Drawing on enumerations of the mitvos and the Rambam My grandfather observes that even if one does not count this commandment as mitzvah unto itself, the goal of many mitzvos is to lead up to this point, arriving at a state of kedusha.
He also points out that the commandment to Moshe to speak el kol adath beney Yisrael indicate that this is not something that applies only on the individual level -- for each person to act in a way that brings him/her to kedusha . It also applies to the behavior of the whole nation, that it should be one of kedusha as a whole.
I noticed today that the penultimate pasuk of the parsha reinforces that reading. It says, 20: 26:
And ye shall be holy unto Me; for I ...am holy, and have set you apart from the peoples, that you should be Mine.
This is a clear reference to the status of the nation as a whole, one that is distinct among other nations.
Related posts: http://kallahmagazine.blogspot.com/2016/05/going-in-ways-of-law.html