Why do churches have steeples? What purpose do steeples serve? Are they put high up on church buildings as some sort of marketing tool so it can be seen from far away or is there some other reason(s)?
Interesting question.  I managed to find an article, The History of Church Steeples, that actually addresses this.

One reason:  It was simply an architectural style that happens to inspire us to look heavenward.


  These early church architects designed grand cathedrals and churches
  that had intricate, soaring steeples. The vertical lines of the
  steeple helped to visually enhance the lines of the church, directing
  the viewers' eyes vertically to the heavens. Obviously, this
  verticality complements part of the mission of the church, to keep us
  in a heavenly frame of mind, but from an architectural standpoint,
  this vertical lift gives the architecture a more graceful and pleasing
  look. The shorter the building, the more squat the appearance; the
  taller the building, the more graceful it becomes. The early church
  believed that the church could communicate the truth of the Bible in
  pictures and symbols to those who were illiterate, such as using the
  picture in the stained glass to tell stories, as well as the steeple,
  which helped by pointing upwards devotedly to Heaven. Therefore, the
  steeple has a dual role in that it helps the congregant in his or her
  spiritual mindset, and the steeple also helps the architect with a
  design feature that enhances the overall harmony of the architecture.


In short, there's no doctrinal significance, or symbolic other than the pointing to the Heavens. It's simply a beautiful style that, once done, was imitated throughout the ages until it became expected.