They are used to improve engine combustion performance at low RPM. They partially obstruct the air path through the intake manifold this causes the air to move with a higher velocity through the opening that is left. This higher velocity allows for better mixing/swirl of the fuel and air allowing for better combustion.
They can be found on both diesel or gas engines and only really if the manufacturer deems it needed. Generally the pop up so solve some type of emissions issue.
Failure modes include anything that you can think of. Stuck open, stuck closed, broken off... For other then a know manufacturing defect there is no specific reason for them to fail.