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Benefit is you have less to budget for when on the cruise. It will clearly state on your travel documents that you have paid, so you will not be charged twic.
This question has been asked many, many times. No you will not be charged again & No there are no benefits in paying in advance. It's all the same price.
We always keep the tips for the end. Some deserve more than it states. Others not as much. Usually we found that our cabin attendant got the full whack wheras the restaurant waiters - not always.