That's exactly what it's for. Modern browsers already optimize request order to grab above-the-fold stuff first. Below-the-fold images are loaded eagerly, but only after that.

Of course, they aren't perfect at this, because in general figuring out what's above or near the fold is equivalent to the halting problem (thanks, Javascript!). They do it well enough for the typical case, though.

Lazy loading is all about the hosting costs.