To my understanding, ”いいですね” means something along the lines of "that's good" or "that's fine." However, the ending particle ”ね” is used to prompt a response from the listener. As I've seen it used in my textbook, ”いいですね” does not anticipate a response. So, what is the function of "ね”?
You'll find lots and lots of resources online about how to use ね. However, the short answer to your question is that ね is a discourse marker used to manage the relationship between speaker and listener. If you listen to how people use ね in conversational Japanese, it's used to establish, maintain, and emphasize agreement between participants in the conversation. It's not primarily grammatical, and its use can't be separated from other ways of managing discourse such as aizuchi and body language.
To understand the difference between いいです and いいですね, imagine this conversation in English:
S1: Hey, I saw Black Panther. It's so good.
S2: Yes, it's good.
Speaker 2's utterance is totally grammatical and understandable, but it doesn't fit the social context. It's sort of failing to catch the ball that S1 has tossed. Every language has lots of tools to manage discourse in this way, and ね is one of the most frequently used in Japanese.