DirSync provides synchronisation between on-premise Active Directory to Office 365 Azure AD.
So here is the scenario where will migrate Dirsync to Azure AD Connect in new Active Directory Forest. We have one of our customers using DirSync on old Active Directory domain. Users & Computers have been migrated to new Active Directory Forest. Now, it was the time to move from DirSync to Azure AD Connet onto the new forest. During the transition, we kept legacy domain dirsync turned on.