Multi-device sites allow your customers to have a seamless experience across multiple platforms. Having a single site for all platforms reduces user confusion as the URL they're visiting is always the same, and the content they're seeing is always similar. The primary benefits of having one site for desktop, tablet, and mobile are:
Because the same content appears on your desktop, tablet, and mobile, your users can find that content on all three platforms. This means that if someone sees a product or service they like on their phone, then later decide to purchase it, they'll be able to find it in the same place on their desktop.
Multi-device sites are built to be usable on any platform, not simply because of the content but the way that content is presented. When visitors navigate the page, they need not pinch or zoom to do so; the content is presented at a usable size and in a usable format.
No need to update content for two different sites. Edit all your content at once without having to worry about mismatching information or data. Also, sticking to one site means it's much easier to troubleshoot versus having to learn two types of platforms.