Inside one document
Styles and templates answer different questions
A style controls how one kind of content looks and behaves. A template bundles styles, page setup, and settings into a starting file. Styles work inside a single document, while templates carry that work into the next document. You need both, in that order.
In practice, that means you fix the heading styles, body spacing, list behaviour, and footer once, in the document you are already editing. Only when those choices hold up do you save the file as a template. A template built on shaky styles simply repeats the wobble faster.
Why templates save time. The dedicated LibreOffice chapter on styles and templates explains what templates are and why they save time. A letter template already knows its margins, heading style, and footer. You focus on wording instead of rebuilding the frame. That saving grows each time the same kind of document appears.