I use compose key sequences to save time writing out long email addresses. For example, I have something like this in my ~/.XCompose:
<Multi_key> <b> <o> <s> <at>: "myangryboss@company.com" # Email of my very angry boss
So I can just type Compose (right alt on my system), bos@ and get his email address. Less error prone than typing out emails manually.
I'm probably not the only one to use compose strings as a replacement to a text expander, but I don't know anyone else who does this.