The first Phantom Cards (Venusaur and Mew) were used in the Mysterious Pokmon Deck from the Legendary Auto Deck Machine in Pokmon Dome
Some interpretations are straightforward, like this, others not so much
Firstly, those planning to sell their Korean cards should take note that KTCG cards dont fetch anywhere near as high a value as their English or Japanese counterparts do
But unless youre milling them out or attacking with an army, you will really be reliant on the Ring-bearer to grind opponents down or perhaps to run them completely out of creatures
Whether or not you should sell a golden chocobo is a tough answer, but if you must have an answer right away, I'd find the card grading service I trust the most, get the chocobo in a slab and hold onto it
Another use is to use a Commander that discards cards or lands for effects that you can tap for, like Lord Windgrace