What Does Amen Mean and Why Do We Say It?
Sure, we can close a prayer without saying Amen, but why would we want to omit a verbal exclamation point which highlights God and His character? Our Lord and Savior is our King, and as His subjects (and as children of God), to proclaim Amen to His truths (and what He has done, is doing, or will do) identifies us as those who love and obey Him.
Source: biblestudytools.com