The use case would be to define one maintenance period that encapsulates multiple tag sets rather than creating multiple maintenance periods.

For example if you were to want to disable alerting for several applications alerts on a specific host and still keep general OS alerting you would have to define multiple maintenance periods.

Alerts to disable in same maintenance period:

1. PostgreSQL Warning alerts.
2. ZabbixProxy Warning/Critical alerts.

Example of maintenance periods as it stands:

1. Disable Host X PostgreSQL alerts (Tags=And event_Application=PostgreSQL, event_Severity=Warning)
2. Disable Host X ZabbixProxy alerts (Tags=And/Or event_Application=ZabbixProxy,  event_Severity=Warning event_Severity=Critical)

Where as if you were able to define 'Tag Sets' then you would define one maintenance period with two sets of tag combinations or perhaps for example:

1. Disable Host X alerts
   1. Tag Set (Tags=And event_Application=PostgreSQL, event_Severity=Warning)
   2. Tag Set (Tags=And/Or event_Application=ZabbixProxy,  event_Severity=Warning event_Severity=Critical)

If you were to use a 'Custom Expression' instead of 'Tag Sets' then you would list all the specific tags and use a custom expression to evaluate for example:

CustomExpression = (A AND C) OR (B AND (C OR D)) 

A. event_Application=PostgreSQL

B. event_Application=ZabbixProxy

C. event_Severity=Warning

D. event_Severity=Critical