Your users have shorter attention spans on the web. When they can't find the information they need, they give up and leave. Grids help create order and make information easier to find, so your users won't leave your site.
In the age of desktops, laptops, tablets, and smartphones, websites need to work everywhere. They need to be accessible and adapt to the device. You don’t want to design (and pay for) two or three different versions of your site for each device type. That's where responsive design comes in.