If you've set up your AMP pages as described in Chapter 1, Ride the Lightning with AMP, that is, with a <link rel="amphtml" ...> tag in your non-AMP pages pointing to your AMP pages, then unless you specifically set something up, the only way to get to your AMP pages is by clicking through a Google search result, or perhaps clicking on a Twitter or LinkedIn link. The point is, you're reliant on third-party services to decide which version of your page will be served to any particular visitor. But what if the user visits your website directly on a mobile device? They will still get your non-AMP, desktop page.
Since you've gone to the trouble of creating an AMP page that's optimized for mobile, wouldn't it be better to get as much value as possible from your AMP investment and serve it to all your mobile visitors, whether Google sent them or not?
To do this, you need some way to determine which visitors are mobile and which are not. This is essentially...