If you use a custom font (anything other than Word’s built-in fonts) in your document, embedding those fonts ensures that whoever views the document sees it the way you intended.
If you’ve ever opened a Microsoft Word document with a custom font that you don’t have embedded, you know that Microsoft Word changes the custom font to your default font setting. That change can mess with the layout of your document and make it look sloppy and hard to read. You can embed custom fonts in your Microsoft Word document to make sure that it retains your formatting when you send it to someone else. Embedding fonts does make document file sizes a bit larger, but it’s worth it in some situations.