You are basically giving them a Creative Commons style license to be able to use your stuff in any way they want, but you retain the rights to the original IP. Like if I download a free song on Newgrounds, I can modify it and make remixes because it's licensed under the CC, and that doesn't change that you own the original song.
Basically they need it so they can convert your files into Google Docs format for simulatenous editing and such, and to convert it into phone/tablet/etc. versions.
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_foolmo_
'Oh please, if foolmo made that analogy you'd think it was picture perfect' - Biolizard28