A hybrid deployment provides the following advantages:
ExchangeOnline and onpremises users can share free/busy calendar data.
Administrators can use the Exchange Administration Center (EAC) to manage both the Exchange Online and on-premises Exchange mail environments.
Administrators can use powerful and familiar Exchange management tools to move users to Exchange Online.
Outlook profiles for users are automatically updated to the Exchange Online environment when the Exchange hybrid deployment and Auto discover are configured appropriately. Administrators do not need to manually reconfigure Outlook profiles or re-synchronize .OST files after moving users’ mailboxes.
Outlook Web App redirection allows for redirection from the on-premises Outlook Web App environment to the Office365 Outlook Web App environment. You specify a target URL for your organization (for example,www.outlook.com/contoso.com).
MailTips, out-of-office messages, and similar features understand that Office365 and on-premises users are part of the same organization.
Delivery reports and multi-mailbox search work with users who are on-premises and those working in ExchangeOnline.
Authentication headers are preserved during cross-premises mail flow, so all mail looks and feels like it is internal to the company (for example, recipient names resolve in the global address list).
If necessary, administrators can easily move mailboxes back to the on-premises Exchange environment.