Most Mac users will have some sort of sleep functionality enabled, it preserves the life of your screen, battery life and a lot of other components in your Mac. However if you're someone who leaves their Mac for long periods of time to do something like video rendering or anything else, this app may be useful.
Wimoweh is an app which allows you to prevent your Mac from sleeping when running certain apps. It puts a little icon in the task bar, from there you can choose which app will prevent your Mac from sleeping. It also allows you to set a time limit so it's very useful.
Wimoweh is completely free and is in BETA so it's not perfect. But from what I've seen, it seems to be working fine for me.