Maybe you were only using 3 GB or 4 GB of your Gmail storage and other space was available for free but that was not available for Google Drive where you may wanted more storage and only option for you was to upgrade your account. Now you can use available 15 GB for Gmail, Drive or for Google+ photos with no specific limits to any of these. You can see how much space you are using. You can also find out how much space is being used by Drive, Google+ photos or Gmail.
I think this is an awesome update that Google got in their system because I didn't used too much Gmail storage but I used Drive storage and when it got full I moved to SkyDrive to make backup of some important files. Now I am thinking about moving my files back to Google Drive because of 15 GB storage for free.