This could as well be answered by asking what do we humans need all the gold for? Granted, it can be facilitated for certain technologies, but the more prevalent answer, especially when looking back at history, is really for nothing, but it's rare and shiny!
The thing is that during history humans have invaded other countries just out of the greedy search for gold, which back then didn't even have a technological use at all and was really nothing but wealth as an end in itself. This is especially prominent when looking back at the history of the Americas, be it the conquest of Southern and Middle America or, more recently, the gold rushes in the North American West, when people killed themselves out of pure greed for the shiny metal.
And I think the aliens from the movie are an allusion to exactly this human greed for gold and especially the gold rushes of the West, which drove people mad. Why should higher technological development of the aliens come with any higher social development? They propagate the same gold-greedy conquest campaigns as the humans did, just at a larger scale, putting us in the position of the conquered people unsuccessfully wondering what they need this gold for.
So the answer to your question would be we're not supposed to know, but it doesn't matter since it's most probably not worth it.