Hi,
I was late, so I took a bus. Here, 'so' expresses a consequence. It is often, although I wouldn't say always, preceded by a comma.
If a particular exercise is little more than a quick review, you can simply give the answers (a comma here ?) so they can correct their own previously prepared works in their textbooks. Here, 'so' expresses purpose. We sometimes say 'so that'. It's like 'in order that'. Generally speaking, it does not need to be preceded by a comma.
Is the choice to put a comma before "so" is an arbitrary one that a writer can decide for him or herself? That's a rather complex question. Here are a couple of simple comments. Use a comma when it helps or is necessary to understand the meaning. In modern English, generally speaking, if the sentence is short, commas tend to be omitted.
Best wishes, Clive