Google already had a reputation for slurping up user data, even in 2008 before they bought DoubleClick. It's a no brainer that a targetted advertising company can target ads better if they have as much information about the users as possible.
In 2008, DoubleClick's over the top, obnoxious advertising (banner ads, punch the monkey, etc.) was slowing down in favor of Google's more discrete advertising and data collection. They may have caught on and turned it around eventually, but Google bought them up first.