Why might you need to separate sand and salt?

Salt may need to separated from sand so the sand can be used to make mortar for construction applications (like a brick wall). Salt in sand will "kill" the strength and resilience of brick or block mortar, and no one with any sense will use sand with salt in it to mix up cement. Bad idea. There may be other reasons for separating salt from sand. And it is probably best done by "washing out" that salt. Salt will, as you probably realize, dissolve in water and sand will not. A continuous wash with fresh water will remove most all the salt hiding in the sand.