In the Modify Style dialog box, in the bottom right corner, there's a check box called Automatically update. If you happen to select this, it could result in surprising and unexpected formatting later, if you apply direct formatting to text that uses this style.
For example, say this check box has been selected for the Normal paragraph style. In your document, you decide you want to emphasize just one paragraph with italic formatting. So you select this one paragraph and click the Italic button. Your whole document turns italic.
What happened? It's likely that every style in your document is based on the Normal paragraph style. Because Automatically update is selected for the Normal style, Word interprets your italic formatting to be desirable everywhere Normal is used.
The default setting for this option is off. Turning it on can be a good thing for styles, such as headings, that have less impact than Normal. Just be judicious about when you select it.