Catholics genuflect in Church in order to show our reverence to the True Presence of Christ in the Eucharist.
Imagine you’ve been transported to the middle ages. You’re summoned by the king to come before him in his palace. Before you go you put on your best, middle ages outfit – which is probably brown. You even wash behind your ears. When you walk into the throne room you know you’re supposed bend one of your knees in admiration and respect of the king’s authority and rank.
Jesus Christ deserves the same kind of admiration and honor that would be due a grand king. Now (back to present day) imagine that you are walking into the throne room of the King of creation, the Lord of Lords, the beginning and the end. When we walk into God’s throne room, the church, out of respect for His glory hidden in the Tabernacle we should genuflect.
Genuflection is defined as 'A reverence made by bending the knee, especially to express adoration of the Blessed Sacrament' (CCC 1378). As we walk into the house of God, a church, we show our adoration for Him by kneeing before Him.
'Come, let us bow down in worship, let us kneel before the Lord our Maker.' (Psalm 95:6)