1. It emphasizes that our worship is not about us. It's about God. The very structure of the liturgy reminds us that we do not come to worship on our own--we come because God calls us to him. It is not our initiation, it is his, and that reality shapes our understanding of what we are doing and the logical way to do it.
2. It emphasizes that worship is something we do corporately. I can sing along to a worship CD and listen to a sermon podcast by myself at my computer (and sometimes I do). God calls us as a people, not just as individuals. I cannot pray with my brothers and sisters by myself. It's a logical impossibility, like a square circle. The liturgy forces me to place myself in community in a very particular way.
3. It trains our understanding of ourselves. A worship service is not something in which I am the consumer and the church staff the provider. God is the recipient, the audience, and we all together are the performers.
4. It prevents us from leaving the hard parts out. And we're not very good performers to boot. We would prefer to think that our offerings are acceptable to God on our own, and our own understanding and effort will get us there. They won't. We have the prayer of confession, because we come into God's royal throne room in pretty sorry shape. We have the prayer for illumination, because unless the Spirit opens our blind eyes and softens our stone hearts, we won't be able to understand what God's on about in the Scripture.
5. It keeps us polite. In this hustle and rush culture of ours, we have lost a good understanding of hospitality. We don't have each other into our homes very much. But imagine, for a moment, going to a friend's house, and simply walking through the front door and starting to unload on them without taking the time to knock, say hi, take off your shoes, and let them pour you a cup of coffee before plunging into the conversation. But often, that's what we want to do with God. We call the church "the house of God," and when we gather, we structure things in such a way as to remember that we are guests (even while we are family members) in his house. Call to worship? Open the door and come on in! Been looking forward to seeing you! Prayer of confession? Sorry my shoes are filthy--it's pouring rain and splattering mud out there. You're right, I don't want your muddy shoes all over my house, God answers. But I've taken care of it. I've got all that you need to get clean right here. And before we leave, the thank you and goodbye and let's set out when we're going to see each other again.
6. It cuts through the cult of spontaneity. As I noted above, we live in a culture which likes to equate "genuine" with "off the cuff." They're not the same, but many people like to think they are. At the same time, these same people want excellence and a good show. So what we end up with are churches that try to project authenticity by making it appear unstructured and unplanned. But the truth is that most all worship services are planned. The musicians rehearse (Some of you may have noticed on the Sundays that they didn't get a chance to.) And the preacher? Even if he's not up there reading from a text, or even looking at an outline, he's thought about what he's going to say ahead of time. He planned. He prepared--however conversational his presentation is and how much he makes it sound like he's just telling you these insights as they occur to him. The liturgy, on the other hand, reminds us that excellence is planned and rehearsed (ask any musician). In fact, the liturgy reminds that our worship services are in fact a rehearsal themselves--a practice session for heaven's music.
Yes, it's easy to let doing the same things the same way become a rote exercise in form. But consider a child at the piano flying through the scales as fast as she can rush through them, hurrying to get through her practice time and on with everything else. Now consider the concert pianist, going through the same scales. Taking her time. Fast and slow. Major and minor, varying the dynamics, never bored but aware of just how much time and effort and practice it takes to get even scales up to the musical standard . . .
* Definition of liturgy: a form of public worship. Ritual. A particular arrangement of services. Most every church has a liturgy--even if it's only "we sing for 30 minutes and then the pastor preaches for 30 minutes.