This is because Drive is now tightly integrated with Gmail, allowing you to embed Google Drive files in email messages.
Perfect for sending large files
Why is this a big deal? It's because email is still the most common way to share files, yet sending larger files as email attachments is fraught with difficulty. They take ages to upload and often they get blocked before reaching the recipient.
Gmail and Google Drive neatly sidestep these two problems, allowing you to send files that are up to 10GB big (that's an awful lot of data).
There's no need to wait to upload a file before you send it because you'll already have it stored in Google Drive. And you won't run into any email server problems because Gmail sends a link to the file, not the file itself.
Keeping it simple
You don't have to find the location of your file, copy the link and then paste it into an email. You just hit the Google Drive button when writing your email, then choose the files to attach. That's it.