advantages in using fins

Hello everybody,I'm davide from Italy,i'm a master swimmer and i have simple question for you. I realize that in your training session you often advise to use fins,well in the pool where i'm use to go nobody use fins and the use is forbidden, but not in saturday morning when there are few people in the pool. Now i'm considering to buying fins.So, can you explain me the advantages in using fins in training session?Thanks in advanceDavide