If cloth was of exactly the right size for your use, and if it had infinite strength and manageable stretch over its entire width, there would be no point to this column, and I'd have to think of something else to write about. Fortunately for me, the world of cloth isn't that perfect, and even if it was, the fabric we'd see wouldn't look enough like period fabric to suit us re-creationists. So sew we must.
Every design is a compromise between weight and strength.
We want to use the lightest cloth possible, to have less weight to carry around, less load on our various methods of transportation, and less strain on the tent's frame. Tears, abrasions, and stretching are the big three tent-killers. Tears happen either because the fabric suddenly sees a lot more strain than it usually does, and more than it's designed to take. To guard against these things, reinforcement must be provided. We use them to beef up those areas where we expect to see more stress than the fabric alone could handle.