"What's the difference?" Good question!
The answer is: there is no difference! It's like: what's the difference between "I have cookies for you" and "I have some cookies for you."
But, in a special situation, this the second might be preferred (that's a "might," not a "must"): if the listener is hoping for or expecting good news, I might prefer to say "I have some good news for you." I might to do this to make the listener more excited, because they are already hoping for some specific good news -- so, if I just say "good news" it's not specific and the listener's mind might not jump to the specific news that he/she hopes for; if I say "some good news," he or she will probably think "ah, this is the news I was hoping for."
So in most cases, there is no difference. But in one special situation, it makes a tiny difference.