Good shoe trees are probably the most important thing that you need in order to preserve the life of your shoes. They should be used at all times when you are not wearing shoes in order to appreciate their full benefits.
The Shoe Snob Shoe Trees are made of alderwood, a wood much like cedar that absorbs the moisture that we constantly subject our shoes to, be it from sweat or rain. This moisture, if not treated properly will deteriorate and disfigure the leather of your shoes, with irreparable consequences; the only way to stop this from happening is to use shoe trees. It's a small price to pay for a long-term benefit, particularly on high quality shoes.