It's still not a seamless experience, you have to click a separate button and deal with permission issues, but it encourages users to upload files to Google Drive and use the Google Drive apps. Gmail tries to solve permission issues by prompting with the option to change sharing settings.
Why would you upload files to Google Drive instead of using attachments? You can send bigger files (10 GB files vs 25 MB attachments), you can edit documents collaboratively, write comments, upload new versions of the files and manage revisions, you can delete the file or change permissions.
Unfortunately, when you send links to Google Drive files, it's more difficult to download the files and you can no longer download them with one click.
There's another problem: Gmail offers 10 GB of free storage, while Google Drive offers 5 GB of free storage, excluding Google Docs/Sheets/Slides files. More free storage would make GDrive more attractive.
Google says that GDrive integration is "rolling out over the next few days and is only available with Gmail's new compose experience".
{ via Gmail Blog }