Attaching a template allows use of styles in that template.
It will not overwrite existing styles unless the box to do so is checked, but see...
What happens when I send my document to someone else? Will Word mess up my formatting?

Attaching a template will not delete any existing styles. It will overwrite styles of the same name if that box is checked.

Every Word document has a normal style, whether it is used or displayed or not. It is there. It cannot be deleted from within Word. The template which was attached also has a normal style. If you told Word to update styles from the template, the template's normal style overwrote the one in the document. That normal style may be set to not appear in the Quick Styles Gallery, to have a lower priority level, or to be hidden.

You may be looking at the Quick Styles Gallery on the Home tab. That is only a small subset of the styles in a document. Using the Styles Pane and the Manage Styles button it is possible to see all styles, even those marked as hidden.

Understanding Styles in Microsoft Word

If styles are based upon other styles, changes to the underlying style will likely change the dependent styles.
How styles in Microsoft Word cascade by Shauna Kelly