That's because the update fixes a load of bugs and security issues with the iPhone operating system. Notable problems fixed include one where the Safari browser could leak your browsing history, while another now-plugged security hole has allowed for some applications to access iCloud files.
There was also a bug that could have been exploited to let malicious apps gain root privileges; basically a level of access that can wreak havoc with a phone,
In total, iOS 15.3 and also iPadOS 15.3 include 10 different security fixes for their respective iPhones and iPads. Somewhat unusual, this update provides no user-facing tweaks or features, seemingly being all focused on security and behind-the-scenes changes.
Nevertheless, Apple champions the privacy and security of its iPhones and software, so this update helps reinforce that position. And thankfully, for those of you not yet on the latest iOS version, there are no reports of the flaws highlighted here being exploited in the wild.
To make sure you’re on the latest version of IOS or iPadOS, navigate to the Settings app on your iPhone or iPad. Then scroll down to the General section. Tap on System Update, and you’ll be able to see if there's a new version of iOS or iPadOS is available. Tap Download and Install to do exactly that and your Apple device will update.
By default automatic updates should be on, but these might not always apply straight away so as to not disrupt your day-to-day iPhone use. So if you have a moment, you can use this approach to give your iPhone or iPad a nudge to update. We suggest you keep all iOS and iPadOS devices updated to avoid security concerns and also ensure you have the most stable version of the software running.