The reason you might be in a hybrid situation is you have on-premises solutions that require having a local Active Directory. (an example may be is that you have applications that require LDAP or a File Share where you still use local AD accounts).
Then you would use Azure AD Connect to ensure that local Exchange accounts (AD Accounts) are also migrated to Microsoft 365 Azure AD.
If you don't have local on-premises applications that require a local AD then it would be safe to say to just cutover completely to Microsoft 365 (Azure AD and Exchange Online).
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