Overview
Installing Windows or any OS can take a lot of time. You have to install the OS, drivers, and software. This can take 1-5 hours depending on the OS and software needed. If you only. have to install Windows on one machine this can be OK. If you need to install an OS on many machines (like the tech people installing ChromeOS on 1,000+ computers), installing the OS one computer at a time is too time consuming. It may also be inconvenient if Windows dies and I need to get it reinstalled quickly. Imaging allows you to reinstall the OS, drivers and software in a matter of minutes, not hours. You can also install the OS across multiple computers simultaneously.
Imaging (or cloning) is basically making a copy of your entire drive or partition that your OS loaded on. If you need to install or reinstall the OS, you just push the image back onto your drive or onto the drive of another computer. Creating and pushing the image to a drive only takes minutes as opposed to hours to install the OS from scratch.
Windows Backup is a tool built into Windows that allows you to backup up files and make an image of the OS so you can recover the OS if it crashes. In order to recover the OS, you also need to create a recovery disk (CD or Jump Drive) that you can use to push the image back onto the drive. This tool can only be used with Windows files and is limited in what it can do. For most users, though, it does all you need. The Windows Imaging page goes into more detail.
Clonezilla is a more advanced tool for imaging. Not only can it make an image of your OS drive, it can image entire drives with partitions, directly copy drives and partitions, and image across networks. The Clonezilla Imaging page goes into more detail.