Styles can be a difficult concept to understand, but are in fact they one of the most productive features of any Word Processor, or indeed DBT in this case.
To quote a simple example, let us suppose we are preparing a book in Word for Windows, which comprises of many Chapters. Each Chapter will normally have a Heading, even if it is only "CHAPTER 1", "CHAPTER 2", etc..
In order to make the Chapter Headings stand out visually, many users would simply highlight the text of a Chapter Heading, then change the size of font, and perhaps the Font itself. Then they might force a page break before the Heading, so that each Chapter begins on a new page.
That is a lot of work to do, especially if you have a book with lots of chapters in it.
On the other hand, you can save a great deal of time by applying the Word "Heading 1" Style to the Chapter heading, by simply highlighting the line, and then applying the "Heading 1" Style.
However, let us say we are not happy with the appearance of the "Heading 1" Style in our document. If we then modify the Style itself, ALL OCCURRENCES of that Style within our document change automatically. Obviously, that is a lot quicker than going though each individual Heading in our document making the changes we want.
When the document is imported into DBT, you will find that DBT has recognised the Word "Heading 1" Style, and automatically applies the DBT "H1." Style to those headings.
Again, in DBT, if we do not like the way the "H1." Style appears, we only have to Modify the DBT Style, and everywhere that Style is used, it will appear as we have changed it.