Google's online cloud storage offerings have been changing quite a lot as of late. It started with Google Docs being transitioned into Google Drive, which was followed by Picasa being merged with Google+. Google would then merge Google Drive with Google+ Photos and offer users a total of 5GB of free storage for their data.Now it looks like Google plans to merge the GMail storage which this offering and upgrade the free storage to a total of 15GB for all users.
GMail users now have a total of 10GB of storage, and its likely that not all users take advantage of all that storage for their email. Since documents and photos have only 5GB to share between them, it seemed like the right choice to combine all the storage into one offering.
If you are a Google Apps Enterprise user, you have an added advantage of having a total of 30GB of combined storage.
source - Google