Hiram’s move is much more significant than this short passage indicates. It was necessary for a king to have a palace for his kingship to be recognised as legitimate. In the Phoenician sagas from Ugarit, after Yam becomes king, skilled craftsmen build:

... a mansion for Yam... a palace for Judge Nahar
... they are building a mansion for Prince Yam
they are constructing a palace for Judge Nahar, a house like...

When Baal conquers Yam, El installs him as king:

At that moment verily the bull El his father,
the god who installed him as king, cried out,
Athirat and her sons,
Ellat and the company of her kinsfolk
cried out:
Now there isn’t a house for Baal like El
nor a court like the sons of Athirat...

Baal complains to his sister Anat and asks her to petition El for permission to build a palace:

And now, no house has Baal like the gods,
nor court like the children of Asherah.
The dwelling of El is the shelter of his son,
the dwelling of Lady Asherah of the Sea.

El agrees that Baal can build a palace to consolidate his position, and it will be magnificent. Anat takes Baal the good news from El:

I have brought you good news.
A house will be built for you like your brothers
and a court like your relatives.
Call a caravan into your house
a convoy into your palace;
the rocks will yield you much silver,
the mountains the choicest of gold,
and a mansion of silver and gold will be built,
a mansion of brilliant stones, even sapphires.

The victor Baal did rejoice,
he did call a caravan into his mansion,
a convoy within his palace,
that the rocks might yield him much silver
and the mountains the choicest of gold,
that they might yield him the noblest of gems...

Yam and Baal gained their kingship through victory in battle yet still were not considered established as kings until they had their own palaces. David wasn’t in as strong a position. His need to establish legitimacy was greater because he had usurped the thrones of both Judah and Israel from the existing royal line of Saul without the benefit of conquest.

The Hebrews originally had "judges" not kings. They instituted kings while they were trying to conquer southern Phoenicia because they saw that the Phoenicians and Philistines (Palestinians) were more effective in battle as they had kings who provided strong central leadership.

Saul was the first Hebrew king and was king of Judah, the territory of the tribe of Judah, which ran from south of Jerusalem up to and including Hebron. Saul was killed in battle along with three of his sons but legitimate heirs to the throne survived. First in line was Ishbaal, another of Saul’s sons.

(The Bible calls Ishbaal Ishbosheth as the Hebrews later changed Hebrew names that included the Phoenician god Baal so that it looked as if the Hebrews had never worshipped the Phoenician gods.).

When David took over as king of Judah, the commander of Saul’s army, Abner, made Ishbaal king of Israel. David ruled as king of Judah for seven and a half years, from his capital city, Hebron. Ishbaal ruled as king over the Hebrews’ northern kingdom, Israel, which covered the Samaria hill country.

Ishbaal was assassinated by two of his army officers but there was still a legitimate heir to the throne, Jonathan’s son Mephibaal, Saul’s grandson, who was crippled in both feet (2 Samuel 9:13).

(The Bible calls him Mephibosheth for the same reason it calls Ishbaal Ishbosheth).

Although Mephibaal was the heir to the throne, it’s unlikely that he could ever have reigned because it seems the Hebrews, new to king-making, had adopted the Phoenician rule that kings had to be without blemish. For example, a later king of Judah, Uzziah, was not allowed to continue ruling when he contracted leprosy: The Lord struck Uzziah with a dreaded skin disease that stayed with him the rest of his life. He lived in a house on his own, relieved of all duties, while his son Jotham governed the country. (2 Kings 15:5). So it’s improbable that the people would have accepted a physically handicapped king

After Ishbaal’s murder David became king over Judah and Israel and nobody made a claim on behalf of Mephibaal. However, Mephibaal had sons, who could have challenged David and/or his successors in the future. So David still needed to consolidate his position.

At this point, Hiram offered to build him a palace. This meant Hiram, the most powerful, richest monarch in the region at the time, recognised David’s legitimacy as king of Judah and Israel. His recognition would have had the same force as a country recognising another country today by establishing diplomatic relations and an embassy.

David was wise enough to forestall future palace coup attempts by taking Mephibaal into his own home and treating him like one of his own sons.

David was 30 years old when he became king, and he ruled for 40 years. He ruled in Hebron over Judah for seven and a half years, and in Jerusalem over all Israel and Judah for 33 years. (2 Samuel 5:4-5)

Hebron was now too far south to be an effective administrative base so David decided to make the more central Jerusalem his capital. Jerusalem was a Phoenician city, inhabited by a Phoenician people called the Jebusites. David attacked the city and managed to occupy part of the eastern hillside outside the walls. This surprisingly tiny area is still called David’s City today and is still outside the city walls. The Phoenicians still lived in the city proper within the walls and much later when David wanted a site to build the temple on, he had to buy land from the Jebusite Araunah at a cost of 50 pieces of silver.