It's not a question of 'why?', but 'why not?'. Soon, a majority of people in the US will access the Internet using a smartphone. More and more this is being done near your place of business. While regular websites may eventually be viewable on all devices, there is no reason to limit your audience.
Google Sites has come up with an ultra-convenient template to make a smartphone compatible mobile site to ensure your basic information is posted, get started at Google Sites - Mobile Websites.
- Include your location and phone number. These things help search engines like Google know you're a real business, and get you better rankings.
- If you're brick and mortar, include your business hours. This is exactly the kind of reason your customers are looking for you on their phone.
- Summarize what you do. Included a limited summary of the main things that you provide.
- Link to your social media profiles. This is a great way to encourage your customers to connect with you and will help you convert sales later on.