It is extremely important to use the shoe tree. To extend the life a pair of shoes implies taking care of the inner part as well as for the outer one.
The shoe tree helps maintaining the shape of the shoes when it is off the foot. Shoes leather it is quite like our skin, the more it bends the more it creases it gets. Keeping shoes always in their original shape will ensure they have a long life. When taking off shoes and leaving them empty for a while, gravity will push the upper part down, as time goes by this would make the shoe losing its shape.
Every time the foot gets in the shoe the leather will get up again. This up and down movement would causes creases. Creases could lead to breaking. Placing the shoe tree in the shoes, just after their off the feet, would eliminate this movement and make shoes keep their proper shape all the time