The search giant — the SE you’re Oing for — gets behind responsive design and serving the same code to both desktop/laptop and smartphone users. Why?
Using a single URL for a piece of content makes it easier for your users to interact with, share, and link to your content, and a single URL for the content helps Google’s algorithms assign the indexing properties for the content.
No redirection is needed for users to get to the device-optimized view, which reduces loading time. Also, user-agent-based redirection is error-prone and can degrade your site’s user experience…
Responsive web design saves resources for both your site and Google’s crawlers. For responsive web design pages, any Googlebot user-agents needs to crawl your pages once, as opposed to crawling multiple times with different user-agents, to retrieve your content. This improvement in crawling efficiency can indirectly help Google index more of the site’s contents and keep it appropriately fresh.
Google also gives some hints on how to make sure your non-responsive site gets properly crawled by the Googlebot. (In other Google news, — Joshua Benton