In most instructions I've read about growing an avocado plant, it says things like the following:
In 2-6 weeks, roots and a stem will sprout from the seed. When the stem is about six inches long, trim it in half.
What's the purpose of doing this?
Trim the roots, yes. This will encourage a denser root system. Trim the stem? I can't see why. An avocado is a tree and no amount of trimming on the stem will make it grow like a bush. Avocado's exhibit apical dominance although not to the same degree as an evergreen. One stem will be more dominant and grow more than the others.