Hard drive for duplicator

I just built a 5 drive duplicator and I keep hearing about attaching a hard drive to my unit. I am not at all familiar with this. This is my first time building a duplicator. Could someone please explain the exact use of a hard drive like would you be able to save movies to the hard drive and burn them straight from the hard drive. I just need to know the pro’s and con’s of this. also good brands to look at and also sizes. Thank you for your help in advance.