A good website does far more than just look good. It needs to be functional and easy to use. Customers like to know where they are and how they can get back. If they feel comfortable, they will stay longer and increase the chances of them purchasing a product.
An increasing number of peope are using mobile devices to search the internet. 1 in 5 mobile internet users in the UK do not use a PC . So having a mobile optimised version of your site for them to use means that you don't neglect a large and growing number of customers.