Transactions in AdTech are different than normal payments.
For example, imagine an ad campaign spending $30k/month at a rate of $5 per 1,000 impressions. The customer may want their budget spread evenly throughout the month, so the software sets a daily budget of $1000. But this really represents 200,000 daily impressions, each of which is a transaction that subtracts from the available balance in real-time. The buyers software is talking to an ad exchange and keeping track of the budget every time an individual impression is won.
To add some more complexity, the impressions are probably billed as second-price auctions, so they aren't all exactly $0.005 each. Some are $0.00493, some are $0.00471, ect. Each one of these numbers is reported back from the exchange to the buyer's software in real time and the buyer is responsible for managing their budget.
This is just an example, but hopefully it illustrates how it can become impractical to account for this kind of thing using something more traditional like PostgreSQL. It would be reasonable to log all the impressions to something like Hadoop for the analytical piece of the software, but there needs to be something more real-time for budgeting to prevent overspending. The big ad exchanges can host hundreds of thousands or even millions of auctions per second, so not turning off bidding can be very costly.
This process of auctioning ad impressions across many buyers through an API is called real-time bidding.