Spinach powder is dehydrated whole spinach leaves, ground down. You get the same iron, magnesium, and vitamins A, C, and K you would from fresh leaves, just without the water and bulk. Most people use it because it is easy to stir into a smoothie, soup, or sauce on busy days. There is no solid evidence the powder does anything fresh spinach cannot do. Think of it as a shelf-stable shortcut for general nutrient support.
Since the powder is concentrated, a small scoop gives you more vitamin K, oxalates, and nitrates than the same weight of fresh spinach. Anyone on blood thinners like warfarin needs to keep their daily vitamin K steady. If you tend to form calcium-oxalate kidney stones, you may want to go easy on high-oxalate foods like this powder.