Halliday & Matthiessen (2014: 534-5):
The fact that the projected clause is embedded as the Qualifier in a nominal group means that it can occur in a range of grammatical environments not open to non-embedded, tactically related projected clauses. This is important in the creation of discourse; one of the central uses of nominal groups with embedded projections is in the representation of arguments, as in newspaper reports and scientific discourse:
There is bitter opposition to his proposal [[that Palestinians renounce their demand [[for more than three million refugees to return to areas inside Israel that were abandoned in the 1948 war]] ]].
Israelis have rejected Mr Clinton’s proposal [[that they give up control of the Temple Mount in Jerusalem’s walled Old City, the holiest place in Judaism and the third most sacred in Islam]].
Boyle’s tentative suggestion [[that heat was simply motion]] was apparently not accepted by Stahl, || or perhaps it was unknown to him.
Here proposals and demands are opposed, renounced and rejected. The contribution to the creation of discourse is further enhanced by the fact that such nouns of projection can be used anaphorically to refer back to propositions and proposals already established in the discourse:
The Labour Party opposed Thor missiles, because, he said, they were out of date and vulnerable and would attract enemy action. That argument did not apply to the Polaris submarine.
The cohesive effect is similar to that created by text references achieved by means of this, that, it:
The talks lasted for three hours. This was a surprise, for they had only been scheduled to last two hours.
(cf. it was a surprise that the talks lasted for three hours with a fact clause; but nouns of projection make it possible to construe the class of projection explicitly.
To be clear, here Matthiessen seriously misrepresents Halliday's model of cohesive reference. It is not the noun of projection (argument) that makes anaphoric reference, but the reference item that precedes it (that). It would appear that Matthiessen has been fooled by the misunderstanding of Halliday in Martin (1992). For evidence that Martin misunderstands the principles on which reference is theorised, see any of the 153 posts here.