Why is praise added to the Lord's Prayer?
Firstly, so that we, asking ourselves from the Heavenly Father for mercies, at the same time give Him fair respect; secondly, so that with the thought of His eternal Kingdom, power, and glory, we are more and more affirmed in the hope that He will grant us what we ask, because this is in His power and relates to His glory.
What does the word: Amen mean?
Means: Truly, or Let it be.
Why is this word attached to the doxology?
This shows that prayer is offered with faith and without any doubt, as the Apostle James teaches (James 1:6).
(“Long Christian Catechism of the Orthodox Catholic Eastern Church.” Reprint published 1913, pp. 69-74)