It correlates with how much a business will be ultimately willing to pay a developer, if they have to.

If I'm making a $1M profit on you, I'll be willing to pay you up to a high fraction of this amount if I have to. Financially, even if I pay you $950K, I'm still making money. Assuming my business scales, I'd hire as many engineers like you as possible, and pay them the same. Why not? I'm making a profit for each one. Great deal for me.

Of course, if you're willing to work for $100k, I'd rather pay you that instead. Hell, I'd rather you worked for me for free!