When it comes to building a new website, having a responsive design is a must-have feature. Your prospects are coming to your site from their tablets and phones, and need to be able to get info on your business without having navigation issues. The site should be responsive to the device it's being displayed on.
Doing QA testing on your website is the way to solve any navigation or display issues before the new site goes live. By checking the site for errors on multiple screen sizes and devices, you'll get a glimpse into what your prospects will see from their phones. You can make decisions on what content is truly important, and what you can hide on mobile as to not clutter up a page.