Responsive websites automatically adapt to the device being used to access them. Therefore, what you see when viewing a responsive website from a computer will be very different to what you see when accessing the same website from a mobile phone. Up to 30% of all eCommerce transactions are being done from mobile devices like phones and tablets. If your website is not responsive, you are losing business!
The Joomla administration area and the user front end are both responsive. That means that you can easily update your website from your tablet or phone. You no longer need to be at your desk to add a new product or deal. With the new Joomla 3 system, you can do so from anywhere when using your smartphone or tablet.