The rating feature feature is standard on the platform for all content types. Rating a discussion thread is voluntary and provides wither SCN members and visitors with feedback on it. This information is visible to other and can be used to generate activities that can be seen in the activity stream, trending content lists and top rated content widgets that, I admin, aren't frequently used on SCN. It's just another way to engagement and provide feedback on content.
Again, it is voluntary just as clicking Like is. If you see something good, you can rate it up and/or like it. If not, do the opposite.