Convenience is the biggest benefit for me. In my line of work, I have to share a lot of information, so being able to create small coded snippets for recurring details is fantastic and makes the process much smoother. Especially since the snippets you create are so customizable from simple text to modifiable fields, formatted text, etc.
I find TextExpander most helpful for LONG amounts of text. I love that in just a few clicks I can frequently pull in an entire page of commonly used text. I also love that it include the original formatting too.