In the movie Cowboys and Aliens what did the aliens need all the gold for?

Why they needed gold isn't really explicitly explained in the film, but in an interview, director Jon Favreau said:

Is there a backstory to why they want gold?

If you look at the mythology of aliens, there's a lot about gold. It's about them coming for gold; whether that's a simplification or not. If you think of Chariot of the Gods, there's this reoccurring theme of gold. If you also say in addition to no faster-than-light travel, that nobody's created transmutation, gold is created in very trace amounts in supernova, so it would be very rare anywhere and rarity would lead to value. It's also a useful, malleable, ductile material that might be used by higher order of technology and if we ever go anyplace else, it would be like Outland. We'd be belters, we'd be mining for resources from other places. So, they would probably have the same logic. Also, most importantly, if works very well for the metaphor of The Treasure of the Sierra Madre and the gold. It's the intersection of where those two things lie and we address it pretty square on. It's either going to be there's here to eat us, which seems silly to me. It's the best choice of all the choices and the one I felt was the most poetic. And then to study us, to get into the whole vivisection mythology of aliens as well, we wanted to encapsulate as many of the images of what people think the aliens do when they're here and then, also, to make it a bit horrific.

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.