There are a few advantages for Google: it only has to bundle a single mail app from now on, the Gmail app is constantly updated in the Play Store, there's a consistent interface for both Gmail and the mail client.
Gmail could already import messages from other accounts using Mail Fetcher, but that's a tool for power users who fiddle with Gmail's settings. "Gmail's Mail Fetcher feature first imports all your old messages to Gmail and then continues to bring in new messages sent to your other account. You can add up to 5 accounts, including Gmail and other email providers."
Email is another AOSP app that bites the dust and is replaced by a closed source Google app, just like Music, Calendar, Browser, Camera and more.