There are all these tasks that we need to do as front end developers. Concatenate and compress our files. Run our preprocessors. Optimize images. Run tests. The list goes on and on. Using different tools for all of them is getting increasingly difficult. Running them manually makes it worse, and easy to screw up.
Grunt is perfect for reining all this in. Maybe you are familiar with tools like CodeKit (covered in this screencast) for doing this kind of thing. CodeKit is great. You might think of it like really specific Grunt setup with a nice interface. Grunt would be a replacement for CodeKit if you need to do more with your build process than CodeKit offers.