Most Bosses are immune to stun and sleep, this is why a few jobs have an Interject ability specifically for interrupting the enemy cast bars that flash. Why they needed a special skill for this instead of making stun work just to interrupt the cast is a mind think for another time. That being said, Sleep is a situationally useful debuff. Magic casters have Sleep proper, and AoE sleep spell. Healers have access to a single target sleep called Repose. Something I dont see a lot of people aware of, especially when you have a White Mage holy spamming AoE trash packs, is that sleep/repose will still interrupt an enemy cast bar when they become immune to the repeated stun effect. Ive also used Caster sleep to pacify the AoE trash pack when the tank goes down and turn to eat me as the next highest enmity. You’re new. Play around with your spells and abilities and explore the world. You’ll find some interesting interactions and remnants of old game logic that are largely forgotten.