Managing a multi-VPC environment can become challenging. Particularly when you have to maintain a very large environment with a full mesh configuration, and on-premises network connections. Transit Gateway simplifies the ability to connect VPCs without peer relationships, connects on-premises environments, and provides edge-to-edge routing capabilities. Let's take a look and what Transit Gateway can do.
Update: AWS Direct Connect now supports AWS Transit Gateway. https://aws.amazon.com/about-aws/whats-new/2019/04/announcing-aws-direct-connect-support-for-aws-transit-gateway/