Often opinions are qualified because it softens the strength of the opinion. Not everyone likes the same things, so sometimes you take another’s point of view into consideration as you give your opinion, or you just don’t feel 100% about something so you tack on a disclaimer. "It was a good taco…for the price." "It was a good movie…if you like suspense." "It’s a cute skirt…if you like yellow."
You just don’t want to take full responsibility for something if you don’t feel entirely sure about it. The range from bad to good is quite wide, so everything can’t be black and white good or bad; most things are just in the middle range of okay. So when someone (who’s opinion I trust) says “It’s good.” And there’s a period to that statement with no qualifications, then that means something—it means it’s actually good.
But this doesn’t happen very often (and it shouldn’t because that would diminish how good the really excellent things are). Not every book or TV show or restaurant is going to be “good” with a period. Most of the time “good” is followed by qualifications that don’t incite excitement. It’s just good…for what it is.