What's the difference between not reporting Apple SMS and reporting spam SMS?

Apple's SMS reporting means that when you receive spam messages, you can choose to report them to Apple. Apple will collect these reports and improve the spam filter based on their information to reduce the possibility of receiving spam messages.

If you receive a text message, you can choose not to report to Apple. This means that you won't send messages to Apple, and you won't improve Apple's spam filter.

In short, the difference is that SMS reporting will help Apple improve its spam filter, while SMS not reporting will not.