Most people keep on thinking why Google bought DoubleClick when they have their own AdSense network for advertisement. That is because most people are not familiar with how the advertisement market is being formed. The large portion of the web based advertisements are in display and text based advertisement where Google is only strong in text based advertisement.
The aim that Google is going for is for them to dominate the display based advertisement market as well. With help from Doubleclick algorithm, Google can optimize the advertisement delivery to each of the servers, effectively delivering relevant advertisements to people in need. Thus, complementary effect that DoubleClick can provide to Google, which can give rise to Google advertisement market yet once again!
Also, in more detail here's why Google bought DoubleClick:
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.
You can find more detail from GoogleBlog on article Why we're buying DoubleClick.