I was always under the idea that Marianne pulled a Xanatos Gambit (http://tvtropes.org/pmwiki/pmwiki.php/Main/XanatosGambit) (minus the villain of course) where she purposely died in order to trigger the events for Lelouch to make a move to over throw her husband.

--Then again this is most likely is because I read too much xxxHOLiC and Tsubasa.

Marianne was important due to just being a wife of the Emperor period, she's a royal family member. However she knew V.V. (obviously given what his and Charles's relation is) and possibly she knew C.C. as well. Most likely her death was either related to Geass or the Sword of Ashaka.