The two are not the same word. E.g.:

1. A valid contract does not exist unless something of value is exchanged between the parties and valuable consideration is given therefor.

2. I think, therefore I am.

The one without the final 'e' is not often used except in legal and other technical language, and means "for that" or "for the thing aforementioned". With the final 'e' is the commonly used one.

Similar to this is 'wherefore' and 'wherefor'. The only usage of the latter that I know of is in the famous line of Juliet in the balcony scene, "Wherefor art thou Romeo?"