Typically governments find it easiest to make an upfront contribution to the capital costs of a project, write this off in their accounts an then ignore any future performance failures.
This is not a commercial approach and leads to the government bearing more of the risk since their money is spent regardless of outcomes.
By making long term performance payments instead, the government can ensure that they receive a long term warranty on the performance of the project. If trains are not being run because of future maintenance issues then performance payments are not made until the problem is fixed at the expense of the private sector partner.
People borrow to buy a house so that they can spread the costs over their full lives and don’t have to save up before they have enough money. Governments should think in the same way when investing in long term infrastructure. But imagine a mortgage which you didn’t have to pay if the roof of your house is leaking or if the windows need repainting. This is the long term risk transfer government is achieving through the P3 model.