Liturgy helps us get comfortable in worship; it lets us break free of the form and structure, or rather, break past them, and to worship more freely and without distraction. I think of it this way: if you've ever prepared a meal in someone else's kitchen, you know how it can be hard to focus on the job. The act of cooking is hindered by the difficulty of finding your way around in the order, setup, and layout of the strange kitchen.
Worship is the same way: if the liturgy is unfamiliar, you're fumbling around looking for what's next, whether you'll need to find something that you don't know where it is, etc. You're distracted from the worship itself, because you're consumed with the form and structure of worship.
This is the case, often, when you visit a church you've never been to before; their liturgy is unfamiliar. It's also the case, sadly, in many churches that want to offer a variety and novelty in each worship service. They won't settle into a patterned worship because they fear the familiarity instead of embracing it. C.S. Lewis called this the "liturgical fidget".
Hughes Oliphant Old offers these thoughts on the value of liturgy:
“In the first place liturgical forms are a good means of teaching the essentials of the Christian faith. When familiar liturgical forms and texts are used again and again, it gives the opportunity to meditate on them and to penetrate their meaning more deeply. When there are well established procedures with which everyone is familiar, it makes it easier to concentrate on content rather than on outward form. Any athlete understands the importance of mastering form. Such simple things as breathing must be done correctly, but this is essential so that eventually they can be done spontaneously, without effort, without thinking about them. The concentration must be on other things. Forms are a means to an end, and if they are constantly changing they obscure the end rather than lead to it.”
~Hughes Oliphant Old, Worship That Is Reformed According to Scripture, Guides to the Reformed Tradition (Atlanta: John Knox Press, 1984), p.162. [HT: Jeff Meyers, in The Lord's Service for pointing me to this quote.]
Liturgy is vital to worship, and we do well to recognize the benefit of it's known and familiar qualities, rather than suspecting them.