Any smartphone, similar to a computer, is made up of hardware and software. The hardware needs an operating system to work. When you buy a new phone, it comes pre-loaded with a certain version of an operating system — Android on phones from Samsung, Google, Redmi, OnePlus, Oppo, etc., and iOS on Apple iPhones.
Google and Apple are continuously working on creating an improved version of their operating systems, rolled out for the phones worldwide. The updated version usually carries new features and aim at fixing issues related to security and bugs prevalent in the previous versions.
Updating a phone is important but not mandatory. You can continue to use your phone without updating it. However, you will not receive new features on your phone and bugs won’t be fixed. So you will continue to face issues, if any. Most importantly, since security updates patch security vulnerabilities on your phone, not updating it will put the phone at risk. God forbid, if anything goes seriously wrong with your phone, then Banking apps would be potentially open to hackers and shall get compromised. Worse would be prying hackers stealing your personal data by exploiting the security holes that you didn’t fix by applying the updates.
Some apps might stop working if you haven’t upgraded them to the latest app version. In short, you have a lot more to lose in terms of convenience and security, if you don’t apply the software updates on your phones.