SNMP traps include additions information along with the trap's name. These are referencing OIDs for lookup, and value updates.

For an example the linkUp trap looks like the following:
SNMPv2-MIB::snmpTrapOID.0 IF-MIB::linkUp
IF-MIB::ifIndex.10001 10001
IF-MIB::ifDescr.10001 FastEthernet0/1
IF-MIB::ifType.10001 ethernetCsmacd
SNMPv2-SMI::enterprises.9.2.2.1.1.20.10001 "up"

The first line is the trap itself (linkUp), after that the interface is told on which it had happened, and the last line is the current value of the given OID, which tells us the new state of the given interface. Such OIDs should be handled by zabbix, so if there is an item for that OID, then it should be updated for the new value supplied by the trap.

The trap descriptions (what OIDs are sent along with each trap) is included in the MIB files, so having a mib file for a trap tells what OIDs to expect, and if such a referenced OID is found in the given host's items, that should be updated on the arrival of the trap. This would be the point of SNMP traps, event-triggered notifications by the SNMP agent, allowing immediate action by the monitoring system (avoiding polling intervals' delays on traps).