By using tags, you create a consistent style throughout a document. You won't have 14 point text in one place and 15 point text in another. Also, each style can be used for many documents. You can change the appearance of all documents that use a given style, all at once, using the Tag Appearance dialog box in any document that uses that style.
At first, you may find the fact that you have to think in terms of tags somewhat restrictive. However, once you understand how to use them, you'll create consistent documents with much less effort than before.