Its not the chest of gold, as the chest of gold is filled with gold coins. The Pirate chest is filled with gems, pearls and other treasures according to the description for it if you look at it.
‘KQ’s usually have a reason for every item unlike some other series like LSL and SQ.’
Actually the earliest King’s Quests were loaded with ‘Treasure Hunting’, that wasn’t directly related to the story itself.
KQ1, had the golden walnut, the golden egg, etc for its ‘treasure hunt’. While they could be used for alternative way to get past certain puzzles, they were not specific main way to get past the puzzles, and you would lose points for using them. So collecting them was mainly to get perfect ‘score’.
KQ2, had the Sapphire Jewels to collect for its ‘treasure hunt’. They weren’t needed to beat the game, but collecting them gave a perfect score and its assumed they were given to Valanice on her wedding day.
The treasure chest was KQ3’s version of the ‘treasure hunt’ from the previous two games, and it existed in order to get a ‘perfect score’.
There is no magic map in KQ5.