According to this blog post Google gives their employees $1 a day to advertise on Google. The intent is to give employees the perspective of an Adwords user in order to improve the product. In addition to giving employees exposure to Adwords, this also has potential to increase the competition in Adword auctions by causing bids to increase and leading to more revenue for Google. Given that Google has more than 10,000 employees worldwide this can have an effect on smaller advertisers.
Other people have written about Google advertising for their own products and giving employees free money can have a similar, although smaller, effect.