As they say... You don't know what you've got until you lose it.
With computers, it's not if the storage fails... It's when the storage fails.
Your storage device will inevitably fail at some stage, and when that day comes, you are going to be wanting to have your backup handy to recover your data.
Some of the critical mistakes we see that customers make is either:
a) They don't have a backup at all.
b) The backup storage device has run out of space some time ago and is no longer backing up new data.
c) They don't have much available space on the computer, so they are only using the backup device to store the information.
d) The backup storage device hasn't been checked in some time to confirm that the device is working properly.
The good news is that this can all be avoided.
A recommended method is to use Cloud Storage such as Microsoft OneDrive, Google Drive or something like DropBox.
The advantages with Cloud Storage is that the files are kept off-site, meaning that if the worst happens and you have a fire, your information is kept safely in the cloud.
Most cloud storage providers offer 5-15GB of free storage space for your documents and photos etc. and if you require more storage, you can simply bump that up to something like 50gb for only a couple of dollars a month. Well worth the investment when something goes wrong.
Final thoughts...
A backup is only a backup if you have the data in more than one place.