You're valuable to free services: you're worth so much they're prepared to give you the service for free just to get you to use it. That might be because they can sell your eyeballs by putting an ad in front of you or because your behaviour on the site provides other useful information.
Sometimes you get an intrinsic reward for that too; knowing which search results you click on lets Google or Bing make the results better the next time someone searches for something similar.
But mostly when you interact, especially with big brands, on these sites what that does is let these companies charge similar brands to advertise to you. That's one reason why Yahoo bought Tumblr and why Pinterest is so highly valued; users there mark very specifically the things they're interested in, and might actually click on ads for, and eventually buy. As the truism goes — if you're not paying, you're the product.
If you look at the number of Facebook users and its annual revenue, as of 2012 you were worth about $5 a year to Facebook. Now you certainly wouldn't want to start paying for Facebook. But wouldn't you like, if not the whole $5 and a service you have to pay for, to get say $2.50 of that?
That's kind of what Bing Rewards does; you get points for searching on Bing and trying out new features like rollover video preview or linking to your Facebook account to see information from your friends that's relevant to a search.
When you get enough points you can trade them in for Groupon, Redbox, Amazon, Starbucks, ProFlowers, RedEnvelope or Tango Card gift cards and coupons, Xbox Live points, credit for Skype or Windows Phone apps, Hulu Plus subscriptions, competition entries, and charity donations.
Sure, you could see it as Microsoft paying you to use Bing and thereby improving the service as well as providing eyeballs to sell ads to, and to use new services so Microsoft gets feedback on how they work in practice.
When you first sign in to Bing Rewards, you can earn points just for seeing what features Bing has, and you get more points when your friends sign up for Bing Rewards. Microsoft wants Bing to get better and better — both to try to get some of the ad money that fills Google's coffers and to power things like the new smart search in Windows 8.1 and the "picks for you" custom app selections in the Windows 8.1 Store.
But you could also say it's Microsoft giving you back some of the money they make by mining your behaviour.
All the online services mine your behaviour and all of them make a business out of it, but few of them give you any of the cash back. Maybe Microsoft's move could be the start of a trend.