Salt can be useful in Phasmophobia to track the movements of the ghost and to complete objectives. In my experience it can also help with getting some ghosts to appear, so that you can get a photo of them.
To use Salt, simply place on the ground around the room or hallway you think the ghost is inhabiting. It's best to take two cans of Salt in the truck in case you place the Salt in locations the ghost is not passing through frequently. This will buy you a second chance to get the Salt in the right place. If your objective is to get the ghost to walk through Salt, you can accomplish this from the truck once the Salt is placed. You don't need to see it to have it count on the whiteboard.