WordPress is web software you can use to create a beautiful website or blog. We like to say that WordPress is both free and priceless at the same time.
They are essential because they know things and think logically. You can build a website without a developer using Squarespace and you can do it for free with WordPress. You can build a form based app with ServiceNow and you can do it for free with (shameless plug for my free tool) Polyapp. You can build video games in Unity. There are similar no-code tools which let you build apps for Android and iOS.