When you purchase Windows 7 from the Microsoft online store, you will get a link to download ISO image file or compressed files. You can only install Windows 7 on the computer to which you have downloaded the file. In this way you can only install Windows 7 on a single computer but what if you want to install it on other computer or reinstall on the same computer due to some reason.
Having a backup of the ISO file or compressed files on a DVD or flash drive is suggested. Microsoft offers Windows 7 USB/DVD Download tool using this tools you can create a bootable DVD or USB flash drive using the downloaded ISO file.