Apple iOS point release betas usually aren’t all that interesting, but the first iOS 10.3 beta contains a big change that, while probably being mostly transparent to the average user, is actually quite interesting.
When you update to iOS 10.3, your iOS device will update its file system to Apple File System (APFS). This conversion preserves existing data on your device. However, as with any software update, it is recommended that you create a backup of your device before updating.
Apple’s developer documentation contains more information about APFS.