Backup is simply copying your data to another location. This location could be on the same computer, or could be in a separate physical location, such as the cloud.
However, in order to get your data / applications / environment back online, the backup needs to be restored. This can require replacing failed hardware, rebuilding an environment, and then copying the data back across. All in all, it can take days – and depending on how business critical the systems are, that may be a crippling amount of down time.