Most cloud storage apps have a few things in common these days. One: free online storage measured by the gigabyte. Two: an Android app. Three: a feature that automatically uploads new photos taken on your phone or tablet to the service. Microsoft really wants you to take advantage of that last one, and to encourage users to do so, they're giving them even more of the first one.
According to this blog post, Microsoft is doubling its already generous 15GB storage allowance for free users if they enable the "camera roll" (automatic photo upload) feature.