Most of the time, you’ll be building your website (or having it built for you) on a desktop-sized device, but over 50% of internet browsing actually happens on mobile or tablet devices. Since these devices have smaller screen resolutions, you need to ensure that your website will be easy for visitors on mobile/tablet to view and navigate.
Problems such as tiny text, boxes/frames that are only partially visible, or pages that require a lot of pinching and zooming to view will quickly put your visitors off. Even if the website builder or WordPress theme that you’re using claims to be ‘mobile-optimized’ or mobile responsive, you should still perform your own checks just to make sure!