Shoe trees cost money. Shoe trees add to shipping weight. And taking shoe trees in and out of your shoes between wears adds an extra step to your busy day. So why do we sell shoe trees, and why should you use them?
First, there are claims that shoe trees help manage moisture by absorbing it, but this seems up for debate by people with a strong understanding of science. After all, luxury shoemakers use varnished woods for their shoe trees, and varnished wood is basically waterproof, no absorption is happening here!
Here's the main reason why you should use shoe trees: they help maintain the shape of your shoes, which means they wear better and look better. They smooth out unsightly wrinkles at the flex point. More importantly, they prevent excessive toe spring from developing.
Someone on Reddit performed an experiment of using a shoe tree on one boot, and not the other in the pair. Within only 6 months, not using shoe trees caused extreme toe spring, which was not only unsightly, but was so uncomfortable to wear that he had to end the experiment. You can read the full details here if you'd like.
So despite the extra cost, extra weight, and extra time involved with shoe trees, we strongly recommend you consistently use shoe trees, for at least 24 hours after you wear your shoes. The longevity and aesthetics they provide your shoes are well worth the money and few seconds of effort!