Why does Brining help food to retain water, but adding salt will draw the moisture out?
For the brine, it's because of osmosis When you have a semi-permeable membrane, like a cucumber skin, water will tend to move from the higherlower solute mixture (the salt water) to the lower higher solute mixture (the water with organic material inside the cucumber). This will cause the cucumber to absorb water AND some salt, until the point where the water in the cucumber is as soluble as the surrounding brine.

When you add solid salt to an item, steak for example, Osmosis is no longer at work. Instead, you're dealing with absorption , a completely different chemical process.