A question about the Lord's Prayer

As familiar as the Lord's Prayer passages are (Matt 6:9-13 and Luke 11:1-4) I was surprised by them last week.

I was thinking of Luke's version, where the disciples ask Jesus to teach them how to pray.

Teachings I have heard about prayer emphasize that it is a relationship with our heavenly Father, we shouldn't think there is a magic set of words or an approved vocabulary, but we should feel free to open our hearts to God.

But Jesus doesn't say this (at least not in so many words) when asked to teach them how to pray. He teaches them a prayer that can be learned by rote. Why?

It is true that if you think about the words, they lead into an awareness of relationship. But why did Jesus give a set of words rather than talk about relationship?