Computers, regardless of shape, size, or form factor, are incredibly complex by nature. The product that the end user experiences often belies the complexity, particularly in regards to the operating system and the quantity of applications installed on it. The way that all of these applications interact with the OS as well as each other inevitably leads to conflicts, necessitating patches and frequent updates.
The conflicts could be as inconsequential as a UI bug or a non-critical function being broken, to critical security vulnerabilities that turns any machine running an unpatched version into an open window for an attacker to compromise the entire network. The WannaCry ransomware outbreak that swept the world in the Spring of 2017 is a perfect example of that: an unpatched vulnerability in an obscure component of Windows that most users were not even conscious of lead to WannaCry spreading globally across thousands of networks and infecting over 300,000 computers.
Most vulnerabilities are not that severe-- at least as far as you can tell. That's why the best policy is simply to patch early--and patch often, particularly on any device that you store sensitive data. People commonly overlook their phones, despite the valued personal photos, contact information, passwords, credit cards, and more that they store on them. Patching a phone is easily as important as patching a computer, so be sure to do that as well.
If you want to read about some current vulnerabilities that were patched in the newest iOS version, check the main article out HERE. It might make you realize just how important it really is!