Shared storage means that users can decide how they want to use it. If you're light on Gmail but heavy on Google+ photos, this means that you can use more free storage towards uploading full-resolution photos on Google+ than on Gmail. Before, you'd only have 5GB (10GB on Gmail) on each service, without the freedom of choosing what to do with the free storage.
Along with the combined storage, a newer, clearer Google Drive storage page is rolling out over the next few weeks. The updated page will feature a pie chart, to help users understand how much of the storage they're currently using and where.
This is great news for people who like all things free, but you can also upgrade your storage if 15GB isn't enough for your needs.
What will you be doing with your new unified 15GB of storage?
Source: Official Google Drive Blog
Image: Google