Cains can be very helpful navigational tools in complex terrain, particularly when they guide travelers to the best route past a dangerous obstacle such as a cliff band, steep gully, or river. They can also be helpful in open terrain where a lack of natural features makes navigation challenging. In well traveled terrain, cairns can help keep people on the proper path and prevent alternate trails or other environmental damage.
That said, some cairns serve no clear purpose and can even cause problems when placed off the preferred route through an area. Superfluous cairns in wilderness are an unpleasant reminder of the presence of people, much the way stacks of stones in a river detract from the natural beauty of the river itself. Like Jason, I knock down far more cairns than I build.