So that players can play large bosses at anytime they wanted except add in diminishing returns to decrease boss farming. Have time triggered bosses reward differently or behave differently… Have triggered bosses have triggers that can increase the difficulty or spawn more of the same boss or other bosses and spawn more minions etc. This would allow players to increase the difficulties of their fight or extend it if they felt they weren’t challenged enough or if they wanted to continue the fight at the current location. limits can be put into place to reduce unwanted results like unwanted farm etc if needed.