It is written, “Raise your hands of holiness and bless the Lord.” Also, a prayer is by raising the hands, as it is written, “And it came to pass that when Moses lifted up his hands, Israel prevailed.”
Raising the hands is regarded as “surrender,” since when one sees that he cannot get what he wants, he raises his hands and says that here he cannot achieve anything with his own hands.
Therefore, when praying to the Creator, we should raise our hands, as it is written (Psalms 63:5), “In Your name I will lift up my hands,” that only the Creator can help him.
Likewise, in blessing, he raises his hands and blesses the Creator for everything that He has given him, since for himself, he is incapable of anything and only the Creator has given him, so he is thankful to the Creator for all that He has given him.