Consider a scenario, when you start a heavy download on your system, and when you are going away from your system, you intentionally leave it running, so that the download gets completed while you are away.
Now, generally what happens is, your computer is scheduled to sleep after a certain time like 20-30 minutes, and while you are going away leaving the download in progress, your system sleeps and this ultimately stops the download, and leaving your system running while away from it becomes useless.
Here is a small app, Insomnia, simply download a small zip file, and run the executable, now as far as Insomnia is running on your system, it won’t sleep.
When done with your download, or whatever operation you wanted to finish, simply exit the window, to make re enable the old power settings of your computer.
Insomnia is a damn simple and useful application, give it a try. Download Insomnia