Think of your website like an apartment; it requires maintenance. You don't just move your stuff in and never touch it again. You have to regularly keep things in order, otherwise entropy comes in and mucks everything up.
If you let it get bad enough, it affects your neighbors, too. Badly. Like cockroaches.
However, instead of having an apartment that smells like mostly-empty pizza boxes housing pests, hackers come to your website and squat. They insert malicious code in your website and wait for an opportune moment to start causing a ruckus — slowing down or completely disabling websites across the Internet, and using your account do it!
How do you prevent it from happening? What's the website equivalent to scrubbing the floorboards? Keep your WordPress® (and other CMS) installations up-to-date.
Outdated software is a huge problem on the Internet. Often, new versions of software get released solely to fix issues that have been exploited by hackers. So, if your website suffers from well-known bug, it's highly susceptible to compromise and needs to be updated.
WordPress even tries to make this really simple for you. Every time you log in, it checks for updates and displays a banner at the top of your admin panel to let you know about them. The process is ridiculously easy and takes less than a minute sometimes.
If you manage multiple Websites, you can also check out Websites like InfiniteWP to make managing WordPress even easier.
"But I don't log in to my Website very often," I hear you say. You don't eat off the floor very often, either, but you've still got to clean it up. No matter how often you log in to post new content on your Website, you should still log in frequently to update its software (along with any plugins you use). If not, you're potentially subjecting everyone on the Internet to a worse experience. It's not the ghastly smell of rotten milk, but a slow website can be just as bad.