When you consider the degree of advanced culture capable of such feats and technologies beyond our comprehension, you are potentially safe to assume they earned that knowledge. By earned, I suspect as a society, they all comprehended and respected the power they controlled. In other words, they didn't blow them selves up by virtue of it being an advanced technology that was simply handed to them.
If your wondering if I'm buying the concept of the plutonium production for their alien overlords space ships, I'm not a fan, but its beside the point. If I'm going to set the "WHY" part aside as something as yet undetermined, I accept the 'asterix' of that theory simply as a post it note entitled "Temporary idea for the underlying question". But really, it doesn't matter at this point, while we're trying to figure out what it is. I'm comfortable putting that question aside as a 'distraction' from the focus which is the reverse engineering exercise. In other words, I'm sold on the "Machine" as opposed to "Tomb", but I don't care who built it yet. But having said that, I do believe like any massive undertaking, it had to have a productive, profit based reason for being created. As an idea, it requires less leaps, and offers necessary respect to such an advanced people.
I could ask you the same question. Do you have a theory? Or, like me, are you comfortable continuing the exploration of the Machine as a purely scientific exercise, without the need to understand the "WHY". To me, that ability is our advantage over the Eqyptologists. Any digression into the archaeological context is a distraction.