I think god must have been working on 'repetition for emphasis'.
This whole account is about how Judah is miraculously saved by Yahweh sending his angel to destroy Sennacharib's army of 185,000 men. Judah is saved because of it's faith in Yahweh in contrast to the northern kingdom of Israel. Yahweh allowed Israel to be destroyed by Assyria because of it's continued apostacy and the evil done by it's kings. History paints a slightly different picture though.
The northern kingdon of Israel had actually prospered under many of it's kings especially the Omrides. It was this prosperity that eventually led to conflict with the expanding Assyrian empire and by the time of the death of Tiglath-pileser III most of the northern kingdom had been annexed and it's people lost to history. But it does make you wonder why Yahweh let many of these kings prosper for so long despite their continued idolatry and apostacy.
When the biblical authors compiled their history of the northern kingdom their aim was not to produce an objective history but a theological explanation for the destruction of the northern kingdom. Each one of the northern kings is judged negatively especially Jereboam I and the Omrides such as Ahab.
When Judah was attacked decades later by Sennacharib's forces it is true that Jerusalem didn't fall. The biblical authors attribute this to Hezekiah's pious faith and Yahweh intervention. But archaeology paints a very different picture. Excavations at Lachish, Judah's second city, show the Assyrian seige ramp and mass graves. An Assyrian relief found at Ninevah shows the full horror of the Assyrian destruction of this city. Further Assyrian records of Sennacharib's campaign and mound excavations throughout Judah confirm the total devastation of Judahite cities.
Hezekiah had fortified Jerusalem against an Assyrian attack including the building of a subterranean tunnel to bring water into the city from the Gihon spring. So although Jerusalem didn't fall Sennacharib achieved all his goals. Faith in Yahweh did not stop large parts of the Shepelah (Judah's agricultural land) being given away to city states of Philistia. Hezekiah was forced to pay heavy tribute to Assyria and a significant number of Judahites were deported. For all the bible's talk of Yahweh's intervention, Assyria was the only victor. Judah had been fully subjugated.
If 185,000 Assyrian troops had been destroyed by Yahweh then none of the above would have happened. And certainly no other Judahite king would ever turn to idolatry again. But is this the case? No. Hezekiah's son Manasseh seduces his subjects "to do more evil than the nations had done" (2 Kings 21:9).
An objective study of the true history of the kings of Israel and Judah compared to the theological history of the bible is important to show why much of the bible was written. The bible authors based in Jerusalem were Yahwists. To them any king who rebelled against foreign influences such as Hezekiah and Josiah were good and faithful. Any king who had alliances or paid tribute to foreign powers was evil and idolatrous.
Israel and Judah's demise was the result of politics and not divine retribution.