Application sets are used to group multiple predefined or custom applications under one named object. Juniper documentation states that an application or application set can be referenced by security policies as match criteria for session-initiating packets. This allows administrators to simplify policy configuration by matching one application-set object instead of listing many individual applications. It also improves operational flexibility: if several policies reference the same application set, you can change the applications in that set without directly editing every policy rule. Application sets do not organize addresses; address sets perform that role. They also do not directly open dynamic ports during a session; that behavior is associated with application-level gateways and protocol handling, not the purpose of an application set.