Trees are essential for good shoes. After a day's wear, your shoes have absrobed a fair amount of muisture. When you take them off and let them dry in the air, then, absent trees, the leather will tend to contract, the front end will curl up like dried lox and unsightly creases will appear. Some maintain that trees help absorb excess moisture as well - I'm not sure I agree, but it's a minor point.
If you can find trees that have been made to fit your particualr shoes, then that is the ideal situation. However, very few companies actually offer those.
You can find inexpensive cedar spring-loaded trees (usually $10-15) in many discount stores, and on ebay and STP. These are usually shaped the same and would not fit, in my opinion, many of the chiseled/elongated lasts so beloved of us.
I usually hunt for vintage hardwood trees on Ebay. I pay on average $15-20 per pair including shipping. Vintage trees come in many shapes and lasts, and over time, once you've accumulated a few, you can match them with your shoe collection.