im not sure if this is built the same way FFXI was, but
+ enmity increases the threat of your actions
There are two types of threat
The temporary kind (like flash) that are typically instant and grab attention. They last for X amount of seconds and then go away.
The second type is more permanent, because it doesnt "go away". This includes actions like attacking or do damaging skills. This type of threat is the same type generated by the DD/Healers (when they attack or heal)
Being a PLD or w/e, you have a natural ability to keep threat because of increased base enmity. Not sure what factor it is, but gear/skills give even more
Also, you lose "threat", by getting hit or having actions taken out on you from the monster. Every time the monster hits you, you lose part of your threat and someone else gets closer to being the target.
Temporary threat is a good way to grab the monster's attention while your threat level is low, so you have time to build up your more permanent threat level through attacks/skills/cures/etc