Examples please

Why would i switch an input to read ground when active and thus require the pull up. An example would be great. thank you.

The reason is because that's the way the integrated circuits (chips) inside the ECU work. The logic inside the chips find it easier to detect a ground connection, than detect a voltage, because then you need a voltage reference to compare with. Resistors are cheaper than comparators. Many inputs can have internal pull-up resistors so all you need to do is switch between the input and ground.