One of the common issues is that liquid gets in along the vent and touches the board since there is no protection, and there are components along the vent that are very important. The onewire circuit is what allows the charger to speak to the SMC so it can give it the go-ahead to allow power through to the rest of the computer.
On older machines, the logic gate that allows the PP3V42_G3H power line through to U6900 which allows communication between the charger and the SMC sits on the edge of the board. This via is very tiny, and always dies with minimal amounts of moisture applied. In the above video, I demonstrated a common fix for this issue, and also proposed alternate methods of restoring the via.
For more information on what the onewire circuit is, here is a video dedicated to just that topic.
As usual, we fix these board issues and many more under our Macbook logic board repair service