I guess this is the official reason
1. DoubleClick's products and technology are complementary to our search and and content-based text advertising business, and give us new opportunities to improve online advertising for consumers, advertisers and publishers.
2. Historically, we've not allowed third parties to serve into Google's AdSense network, which has made it hard for advertisers to get performance metrics. Together, Google and DoubleClick can deliver a more open platform for advertisers, and provide the metrics they need to manage marketing campaigns.
3. By combining Google's infrastructure with DoubleClick's knowledge of agencies and publishers, we can create the next generation of more innovative ad serving technology, one that significantly improves the efficiency and effectiveness of online advertising.
4. To manage ad inventory, some of the largest publishers use DoubleClick DART for Publishers – but a good portion of it goes unsold. It's our view that the combination of DoubleClick and Google will help these publishers succeed by monetizing their unsold inventory.
Personally I think it was just to stop the competition getting their hands on it