Most WADS allow you to target either desktop or mobile. Some let you target specific operating systems, or makes of smart phones, tablets and desktop computers.
Sometimes it's a mobile only offer like an iPhone video game install. In this case you would obviously block anyone using a desktop computer, Android, Windows, and everything else using an operating system other than Apple.
Another example is virus protection software for the PC. In this case you would block anyone on the Mac, or any mobile device from seeing your ads.
A lot of mobile traffic is spent during commutes, doing social media browsing. Mobile users want quick access to facts, quotes, news, recipes and product reviews, often while shopping in physical stores. Then at checkout, the mobile is used as a payment system, instead of having to use a card.
Desktop traffic is heavier during office hours. Desktop users do a lot of research and make most of the purchases. The time spent reading landing pages is three times longer than mobile. And according to my experience, desktop users tend to make between 60-70% of all purchases when an order form, shipping info, and a credit card are required.
So other than app installs and in-app purchases, desktop is still king of the hill when it comes to purchasing. Desktop converts on average around 2.8%, tablets around 2.4%, and smart phones a mere 0.8%.
This is why I block smart phones on most PPC campaigns. There's no sense in paying for those clicks, if they're going to convert so poorly compared to desktop.
Yes, some would argue that people might search on the mobile, then walk over to purchase on the desktop. That may be ok for a retail store when you own the domain name. But when it comes to affiliate marketing, my affiliate link that they clicked on using their phone, isn't going to magically transfer to their desktop computer.
The bottom line is, unless it's a mobile only campaign for game installs, or something like that, you may be better off shutting them off. Display your PPC ads to desktop computers, and have a far greater chance of getting the clicks that convert into sales.
Attention is the currency of modern society.
The scarcest media resource today isn’t attention, but intention.