Primarily for security. Outdated software leaves your website vulnerable. WordPress is a popular target for hackers because of its enormous popularity. It’s also important to understand that in most cases malware and hackers are not specifically targeting you. Typically it’s a script that’s programmed to target hundreds or thousands of websites, often looking for a “backdoor” into your website. This backdoor is very often caused by outdated software such as WordPress plugins, themes, or WordPress itself (WordPress core).