Think of style guides as a rulebook.
Not to limit, but to keep things consistent – when to bold, italicise, or have headers break up text etc.
It’s important because we look for patterns.
Break the pattern and we get jarred, like going over a pothole. Also, consistency creates signposts that demarcate new sections, place names, even hyperlinks!
What goes inside a style guide depends on what it’s for: brand identity, video, magazine or website? It might cover editorial-type styles – from grammar to how to write a post – or it might cover colours and dimensions.
The contents might change, but remember: a style guide is for consistency.