There are a couple of differences in functionality when using a synced and a non-synced account in Engage.
With a non-synced account, you can perform most basic functions in Engage including sending an email, include an attachment, send a group touch, and automatically recording a touch for each email sent from within the application.
In addition, several features are available for synced accounts. A synced account has the ability to automatically record touches with every email sent outside of Engage. You also have the added ability to sync your email contacts.