The motivation is a monthly paycheck from an ad network without having to build a real audience for your website. If you can generate fake views and clicks at a lower cost than the network pays you, and convince the network the bot-generated activity is genuine, you're printing money.
> What is the benefit for bots that are hitting other people's sites?
That's the "convince the network the bot-generated activity is genuine" part. A site where all the "people" clicking ads are never seen on other sites stands out like a sore thumb. Having a bunch of background benign-looking activity lets you hide other kinds of fraud, too: http://krebsonsecurity.com/2014/07/service-drains-competitor...
By that, I mean, who's paying for bots to generate this activity, and what do they gain by it?
edited :)