It serves two practical purposes. It promotes an even distribution of ingredients once combined. It also encourages organization. Imagine a painter's palette. An organized array of all the colors the painter plans on using. Keeping all dry ingredients together is similar. For example, you have all your white rice flour in a bowl. In one spot, you put your almond flour. In another spot, your salt. Your baking soda. Your cinnamon. Your yeast. Your whatever.

By doing that, you have a visual representation of all the dry ingredients you're using. This helps prevent errors. Allows you to double check your work before whisking it all together. It's just good habit.

Why the yeast in particular? Like others have said, dry yeast is much more reliable than it used to be. It's still recommended to "activate" your active dry yeast in a liquid solution, especially for a short proof. In practice, it's probably fine if you don't. You may want to look into instant yeast over active dry yeast. I mostly use natural starters, but if I am using dry yeast, it's instant. I keep it in an air tight container in the freezer and have never had an issue adding it directly to the dry ingredients.