iOS 10.3 sounds like a minor update, but it actually contains a pretty big change. Apple is using this update to change the file system that iPhones and iPads use from the 30-year-old HFS+ file format to the new Apple File System.
Apple File System is a huge improvement over HFS+, bringing with it features such as optimization for solid state storage, strong encryption, better file time stamping (down to the nanosecond), and many storage saving improvements.
Apple plans to move macOS, iOS, tvOS, and even watchOS to Apple File System.
But the two file systems are not compatible with each other, and once you upgrade to Apple File System, there's no way to convert back to HFS+ without blowing away all your data and reformatting the storage.
What this means is that when iOS 10.3 is installed, every file stored on the device will be converted from the old format to the new format. And while we can expect that Apple will put a lot of effort into making this conversion process as smooth and painless as possible, upgrading a file system always opens up the possibility that something can go wrong.
And if something does go wrong, data loss is a real possibility if the upgrade process has started converting files.
Which is why you need a backup.
There are two ways to back up your iPhone or iPad.
#1. Create a backup using iTunes
Create a secure, encrypted backup of your iPhone or iPad
#2. iCloud backup
Go to Settings > iCloud > Backup, then turn on iCloud Backup.
When you sign up for iCloud, you automatically get 5GB of free storage. However, most people are likely to need more. You can buy upgrade packages of 50GB, 200GB, 1TB, or 2TB of storage, from $0.99 a month.