Nate Silver, "The Other Advantage of Opting Out":
If you simply know someone's name and either their address or their telephone prefix, you can determine their location (with 100 percent accuracy), their gender (with virtually 100 percent accuracy), their race (with high accuracy), their income level (with fairly high accuracy), and possibly something about their age. By cross-referencing other databases, you might also know something about their past voting behavior, and their level of engagement at different phases of the campaign. You have even more information if you know their employer and occupation, as required on FEC disclosure forms.
So apart from greasing the wheels of the campaign, the other thing that private fundraising provides a campaign is an incredibly rich source of data: essentially a real-time, n-dimensional focus group of its supporters...
Fundraising data isn't the only way to collect this sort of information -- but it's a particularly efficient one, especially for a campaign that relies heavily on Internet financing and has thousands of donations rolling in each day. If it makes the difference of being able to react to a dangerous situation 24 hours sooner than you might otherwise, or seeing an extra move ahead, that's how campaigns are won and lost.
Someone could write an entire article (maybe even a book) detailing all of the ways political money is different from regular money. In ordinary contexts, money is anonymous; in political contexts, money is high-information demographic data. In ordinary contexts, money is transactional; in political contexts, an explicit quid pro quo transaction is usually illegal. In ordinary contexts, money is silent; in political contexts, money is "speech." Und so weiter.