There are two levels of device targeting. The first is to target a specific type of device, desktop-laptop, smartphone or tablet and soon others types like connected TV.
Device type targeting allows advertisers to personalize their messages and creatives or to prevent to deliver to an audience not interesting enough. For instance, mobile phone users are not an effective target for too complex offers needing a lot of reading and navigation.
The second level is device model targeting that allows advertisers to target specific phone / tablet brands or versions for mobile campaigns. It permits to adapt to specific phone capabilities. It is precious for app and game designers, accessories vendors and, of course, phone and tablet manufacturers.
Models targeting on AdWords platform:
Device targeting option on Twitter: