Is it important when praying to Creator to end the prayer with the word “amen” which is customary in many religions, and if so, why?

This is simply a cultural convention that grew from the practice of the day to make a clear declaration of sending and receiving, to state Creator’s name upfront and then end with the closing word representing the intention and meaning that the thought is complete and being launched as a message to Creator. This is quite unnecessary because Creator always knows when one is addressing the Almighty and will always hear their voice, their thoughts, and the intention behind it. When they are finished, it is apparent to us, and so we need no additional signal, as would be the case in a telegram to ensure the entire message has been conveyed and something wasn’t lost in the transmission, so the ending salutation signals this. But in the case of reaching to the divine, there is always reception. There are no problems from interference of any sort. The same is not true in receiving feedback from Creator, but that is a different matter entirely.