On the free OneDrive service, enabling camera roll doubles the free storage limit from 15GB to 30GB. This also applies to their Android app.
The camera roll is quite a nice feature as it uploads the photos and video clips as they are taken (or over Wi-Fi only) and syncs them to a 'Camera Roll' folder on OneDrive on a PC.
The Google Drive app appears to be missing this feature! While the Google+ app provides automatic photo backup, it's missing the ability to sync these photos to a PC or Google Drive.