For a backup

1. So you won't lose your stuff. Backing up any type of data can save you a heartache later on should something horrible happen to your device.
2. They watch your back. Having a backup when doing something potentially dangerous (or simply scary) can make it easier for you to focus on whatever you're trying to do. After all, if you fail, someone is there to help.
3. In case the original gets ruined. Be it schoolwork or a batch of cookies, having some extra just in case something happens can be great. Especially with cookies, as you get to eat the extra if they aren't needed.
4. In case your first choice isn't available. A lot of things require that you pick between multiple options. Sometimes, your first choice falls through, and you'll have to pick something else. This is when having another choice already in mind saves time.
5. So you can share it. If you have a backup that you wind up not needing, then there is a possibility that you could give it to someone else. Extra cookies, for example.
6. In case the first one wears out. Having a backup copy of your favorite book is awesome, as you don't have to worry about it being out of print should pages start falling out of the first copy.
7. So that you can keep working. If you're constantly updating your backup and always have access to it, then you never have to stop working on whatever it is.
8. So you can have an opportunity to rest. Having someone who can fill your position for a time can provide time for you to relax.
9. They can save the day. When something is overwhelming your group, having others come to help can is awesome.
10. So you don't have to worry. No matter the kind of backup, simply having it available can be a great relief.