As there are vexed questions in the use of spelling (judgment or judgement?), punctuation (should a capital letter follow a colon?), and grammar (prepositional use?) a Style Sheet helps the editor impose consistency on a manuscript.
Then there are decisions to be made about how to refer to clock time, or how to set out dates, or numbers, abbreviations, people’s titles, and many other points of detail you might never have had to think about before.