Well... You use shoe trees to help preserve the shape of the feet area once you've worn your shoes/boots.
When you wear shoes of any kind, the leather gets warm and moist; since leather is a natural material and is relatively easily shaped by heat and moisture, they won't keep their original shape unless you force them to. The shoe trees keep the shoes stretched/pulled up so they don't loose their shape. That's it. And you don't need to keep them in there indefinitely, just for 24 hours maximum if you've used your shoes an entire day. That means that you don't need shoe trees for every single pair of shoes you own.
For boots, you might want to use boot shapers as well, they basically keep them upright so you don't get nasty creases. Like some others have noted, you can just stuff a rolled-up magazine or something in there... And keep it there until you use them the next time!