I think this sentence states a widely accepted popular opinion rather than a personal opinion. Therefore it could function as an argument. That's why it uses -dir. But İtalya çok güzel bir ülke. would be correct too.
It could also state a personal opinion and then -dir means you are very confident about your opinion and you try to argue with your personal opinion, which many people unfortunately do.
It has a different meaning. "Beautiful" (güzel) generally means "nice to look at"
Just "nice" (I think the closest is nazik in Turkish, but I'm not sure of its connotations) could be for any reason, the weather, the culture, the people, the food ... or all of these.
You can say "beautiful" for non-visual things but it's much stronger than "nice". She sings nicely means her singing is pleasant. She sings beautifully means her singing is very enjoyable because it sounds very nice.