It is a bad idea to delete built-in styles.
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From a methodological point of view, their names have been chosen to suggest their usage (Text Body, Heading n, Quotations, Addressee, Signature, etc.). This encourages a systematic and rigorous use of styles. The names don’t describe visual formatting but the semantic content of paragraph or sequence.
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By default, built-in styles are organised hierarchically in a tree-like structure. It is enough to modify a style at an intermediate position (not a leaf) to have this modification propagated to all dependent styles. This feature is usually disregarded but is tremendously convenient when setting up a coherent family of styles.
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Though this has usually no importance for a standard end-user, built-in style names automatically translates into the UI language. Thus when we talk here about Heading 1, the discussion is valid for Titel 1, Titre 1 or Titulo 1.
Built-in styles can be modified to fit your needs. Don’t hesitate to do it. It is easier than replacing them all. Even if you don’t use a style, its presence acts as a reminder for a possible miss in your formatting.
Last and most important, eliminate direct formatting (manual formatting without styles) in your document. This possibility is offered as a quick-starter or experimenting device but is the source of all evil and causes many subsequent formatting problems. I know this is the sole formatting possibility in the well-known proprietary competitor because of its lack of style families, but there is no reason to keep on applying bad principles in otherwise clean applications.