What effect does syntax have? I know syntax can help to create a conversational and informal tone, but doesn't it do something else as well?
Syntax is just sentence structure.
Oxford Dictionary defines syntax as "the arrangement of words and phrases to create well-formed sentences in a language." Your syntax, or sentence structure, greatly affects the tone, atmosphere, and meaning of your sentence. It can make something sound more formal. And of course, bad syntax can completely confuse the reader.
The main goal of syntax is clarity. One sentence can have several different structures, but there is always one that is clearer than the other. You never want awkward or wordy syntax because it confuses the reader, and consequently detracts from your credibility as a writer.