Why do Christians sing hymns in church? Given your answer, what kind of hymns are most effective?

This is one of their acts of worship to God. Its tradition goes way back to the first century A.D. Emperor Trajan (ruled between AD 98-117) received intelligence about the Christians that their normal practice was to attend a meeting before daybreak on the first day of the week (what we call Sunday), at which they sang a hymn to Christ as a god, and took an oath to abstain from crime rather than to commit it; thereafter they dispersed but met again later to eat ordinary food. Trajan received this information from Pliny the Younger, governor of Bithynia in Asia Minor, who was seeking confirmation about whether to execute Christians simply because they were Christians, or whether they could be saved by recanting.

Notice how those first century Christians were singing hymns to Christ as God? Praise of God is what Christians say and do, sing and enact. It is on a par with praying. It is part of their "sacrifice of praise" - Hebrews 13:15 saying, "Through Jesus, therefore, let us continually offer to God a sacrifice of praise, the fruit of lips that confess his name. And do not forget to do good and to share with others, for with such sacrifice God is pleased."

That covers your second question - the kind of hymns that are 'most effective' - ones that praise and confess our Lord, in conjunction with acts of sacrificial service.

I would just add that the matter of singing God's praise runs right through the whole of the Bible, back to creation itself, "when the morning stars sang together" in praise at God's creation (Job 38:7). And singing God's praises is a theme of Revelation, where the angelic hosts do that right now and will - into eternity. Read Revelation 5:9-12 & 14:3 & 15:3 I reckon we Christians who love to sing God's praises in church are getting into training for the choirs doing that in Heaven!

Tradition. The Psalms in the Old Testament were sung. And it's a way of praising God. I have idea what you mean by "effective", but our hymns are ancient and beautiful.

Hymns and praise music is not supposed to be "effective" it is for praising the Lord in song.

They use poetic images and rhymes to help with understanding gospel themes and aid in the memorization of select scriptures.