Global Styles allow you to create combinations of styles and apply this easily and efficiently across multiple elements. A simple example to illustrate this feature: let's say you want to create a simple, but very nice button with a red background and white text color. And you are planning to have multiple such buttons on different pages in your site or funnel. Now, you could go from button to button and set the background color as well as the text color. Or, using global styles, you can create the desired style once, save it as a global style and apply the saved style to as many buttons as you'd like.
Through magic happens when making changes to one of your global styles; when updating an existing global style, the changes are automatically applied to all elements within your site using that global style.