(in formal usage, especially legal usage) for that or for it
"ordering goods and enclosing payment therefor"
"a refund therefor"
Therefore, for that or this reason or cause.
"She decided to quit her job therefor she felt unappreciated by management."
Therefor originated from combining the word "there" with "for." This process, known as univerbation, merged the two separate words into a single unit.