Explanation/Reference:
Explanation:
Azure AD Connect sync Staging mode can be used for several scenarios, including:
High availability.

Test and deploy new configuration changes.

Introduce a new server and decommission the old.

Have a spare standby server - staging mode
If you have a more complex environment, then having one or more standby servers is recommended.
During installation, you can enable a server to be in staging mode.
SQL Clustering would be needed for high availability.
Note: SQL High Availability
If you are not using the SQL Server Express that comes with Azure AD Connect, then high availability for SQL Server should also be considered. The only high availability solution supported is SQL clustering.
Unsupported solutions include mirroring and Always On.
References: https://azure.microsoft.com/en-us/documentation/articles/active-directory-aadconnectsync- operations/