The medrash says that when G-d separated the waters above the sky and the oceanic waters the lower waters were upset. They were jealous of the heavenly waters. So G-d, hearing their gripe, gave them something special. This is why salt is put on sacrifices, which rise to heaven.
Why didn't G-d simply command water itself to be put on every sacrifice? Rav Yaakov Kaminetzky's insightful answer is this: Salt is what's left when water evaporates. Even the lower water is holy because it rises to heaven. G-d is pointing out though, that what remains of water- the salt - after some goes up, is also holy.
Salt is used on holy korbanot - sacrifices - special acts of drawing closer to G-d as a reminder that what might seem exclusively earthly has holiness in it and can be directed heavenward as well. The take home point here is that there are parts of our life that are clearly the "going up to heaven" pieces. But the parts that seem left behind are holy as well. It's up t us to treat what seem to be side points and subtexts, the residue of the holy, as holy too.
May we be so blessed.