The expression is used to both add additional information AND usually to add emphasis.
As an example, let's say that you called your boss stupid and he fired you (terminated your employment). When you come to me to tell me about it, I could start listing the bad things about the situation like, "Well, you will find it hard to get a new job since he won't give you a good reference. Your plans to buy a car are destroyed since you have no income. You won't be able to pay your rent this month. Not to mention, you still owe ME money!!"
If you don't use the expression, nothing is lost in conversation.
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The expression is used to both add additional information AND usually to add emphasis.
As an example, let's say that you called your boss stupid and he fired you (terminated your employment). When you come to me to tell me about it, I could start listing the bad things about the situation like, "Well, you will find it hard to get a new job since he won't give you a good reference. Your plans to buy a car are destroyed since you have no income. You won't be able to pay your rent this month. Not to mention, you still owe ME money!!"
If you don't use the expression, nothing is lost in conversation.