On the very first site I built I did a ton of customization to the theme and a few months later when I updated it, I wondered what happened to all my customization. Well, now they tell you to use a child theme or you will lose any and all customization when you update your theme, but they didn’t then.
Obviously a child theme is important because you don’t lose any theme customization but there’s more. Not updating your theme is a huge security threat. I have seen many WordPress sites that were up to date that had an out-of-date theme that were hacked or had redirect code. For example: I came across a website that was running WordPress 3.9 but had a theme that was very outdated. On a computer the site loaded with no issues but when you pulled it up on a mobile phone it redirected to a porn site. I removed the hack and told him that he would need to update his theme or this would likely happen again. I went to the site maybe two weeks ago and sure enough, it redirected to a porn site on my phone again.
He had a lot of customization to his theme. So really, I don’t blame him for not updating the theme. I also don’t blame him for not setting up a child theme. Because here is the key to child themes: you need to set them up before you do any work on your website. Otherwise, you will have to start all over.
There are a lot of helpful articles on creating a child theme, they’re really pretty easy but the main point I want you to take away is that as soon as you pick a theme you need to create a child theme.
Helpful articles on creating a child theme: