Though beautiful crystals can be grown from a variety of solutions, we are just going to look at salt for the moment.
Why would one bother to grow any crystal, much less salt crystals?
'Because you can', immediately springs to mind in response, but that's not the only reason.
If you have, say, copper sulphate in solution and want to extract its purest form you might use this method.
You can, of course, simply evaporate the solution but this may leave behind any other elements that may exist in the solution.
Looking at salt crystals is extremely important because its unwanted growth can cause enormous and costly damage to buildings and building materials.
Have you ever noticed, near the ocean, that some sandstone and bricks are all flaking away?
Salt laden moisture in the air soaks into the material, if it is not correctly treated, and the crystals grow inside the stone or brick, popping little pieces off as it does so.
This will occur in bricks that have not been 'baked' at a high enough temperature. All that aside, it is a pretty simple and inexpensive project for school.