It's not just regular Google users that get unified storage, as Google Apps users now get 30 GB shared between Google Drive and Gmail. For Apps users, the change isn't that big, they already had 25 GB of storage for Gmail.
What's more, this isn't technically free storage, since Apps users pay for the service. There is the option to buy additional storage, as needed, from Google.
One benefit of the unification is that Apps users no longer have a limit of 25 GB for Gmail. Any additional storage they buy can be used by Gmail as well, so there's technically no more limit to how much space they can use.
The 30 GB will be shared between Docs, Gmail, and Google+ Photos, for any profiles created using an Apps account. However, native Docs, Sheets, and Slides documents don't use any space and neither so photos up to 2048 x 2048 pixels, stored in Google+.