First of all, note that your first sentence Julie et Marc sont de bons amis could mean two things:
- Julie and Marc are good friends “together”.
- Julie and Marc are both good friend of the people speaking.
However your second sentence Julie et Marc sont bons amis can only mean they are good friends “together”.
Other than that, these two sentences are pretty similar if you’re using the “friends together” meaning.
If you’re wondering why “de” is being used, you could translate it to “some”, meaning they are good friends, but they are not the only possible friends they could have.
The following doesn’t sound nice in french, but for the example: Julie et Marc sont les bons amis (à avoir) would have added a nuance that they are possibly the best friends you could have.