Kale has risen in popularity over the past couple decades, making it a hallmark of healthy eating. And not without reason -- kale is a highly nutritious cruciferous leafy vegetable. It is an excellent source of vitamins K and C, providing over 100% DV in 3.5 oz. (1 bunch of kale is approximately 8 oz, but this will vary between different bunches) It is also a good source of Vitamins A, B6 and B9 and Calcium.
As the chart below shows, most nutrients are at higher levels in the raw form of kale, than in kale that is boiled and drained. However, raw kale can be hard to digest and cause intestinal discomfort, so roasting, lightly steaming, or sautéing will render the plant more digestible, while preserving more of the nutritional content. Also, if you do boil kale, save the water to use in a soup, since some of the nutrients have leached into the water itself. The easiest way to do this is adding chopped kale leaves right into your soup!
The stems of kale are quite edible and delicious, but are chewier and require a more through cooking. I like to separate the largest part of the stem from the kale, and cook it separately from the leaves. You can boil the stems to make veggie stock, add them to a soup, or chop, steam, and toss with butter and salt.
(Nutrition chart used from Wikipedia, shared under Creative Commons License CC-BY-SA, accessed on 2/16/21)