Why do you need to distinguish between mens rea and motive?

Motive and Mens Rea are not the same. Mens Rea is the blameworthy state of mind of the Defendant, whilst motive is just the reason and is irrevalnt in criminal liabilty. For example: D. stabs V. with the intent to kill, so he can marry his sister. The M.R 'intent to kill' is needed whilst the motive 'marry his sister' doesn't matter