A trade mule is an extra character whose sole purpose in life is to craft things for your other characters.

For example, if your main character is a leatherworker/skinner, and you’re looking for a way to increase your profit margins on Toughened Leather Gloves, it would make sense to create a trade mule who has herbalism and alchemy as his professions.

With two characters working side-by-side, you can get all of your Elixirs of Defense for free, and greatly improve your profit margin on Toughened Leather Gloves.

In fact, you don’t even have to level your trade mule above level ten in order to get your alchemy skill high enough to make Elixir of Defense.