1. To lay a foundation from an earlier and informing text – Perhaps your New Testament passage leans heavily on an Old Testament passage, so you read it for the sake of familiarity once you are explaining the connection during the sermon.
2. To avoid giving away the “tension” when preaching a narrative – Perhaps your sermon reflects the tension and resolution of a good narrative, so you want to avoid a recent reminder of how things work out in the end. So you read something vaguely supportive of a theme in the sermon.
3. To support an earlier “phase” of the service – Perhaps you, or the worship team, have designed the service to flow through a greater sequence, of which the sermon is only a part. Consequently the reading of the Bible earlier in the service is intended to fit with the songs around it, rather than as the sermon text.
4. To be appropriate to the day in the church calendar – Perhaps it is Trinity Sunday, or Pentecost, or Reformation Day, or whatever. So you read a relevant passage, but then proceed to preach a message that may be only indirectly connected, or may be completely unrelated.
5. Because you were assigned the text, but basically intend to morph the message into a passage you are ready to preach – So you read the assigned text, but then do a couple of swift moves in your introduction to move into the sermon of your choice.
There may be other reasons too. I tend to see the first three as being more legitimate rationale for this practice than the last couple, but that is not the point of this post. What is?
If there is any possibility of doubt or confusion for the listener . . . explain! Seems simple enough, but it is amazing how often we lead services and expect people to grasp the master plan of our clever service design through some sort of mind-reading or osmosis. It may not matter if people get the full riches of your artistry. On the other hand, if they are confused by it, then it is probably counter productive.