Thanks for the question. The answer is in the name itself as MultiPass Keying. Not all keyers are equal in any commercial or open-source compositor. I have demoed this multipass keying workflow example because 90% of the time you will not even green screens because of lighting or keys colors. One node does not fit all based on what the scene or shot requires. See attached images below to see different lighting, colors of wrinkled fabrics/materials, or layers for ground and wall backings. Some keyers may get all of the green but at the risk of damaging your core matte edges, wrinkled fabrics, hair, and flesh tones if not careful. You can use roto nodes to Gmask out your screen garbage and that is fine. You will have to do a lot of roto keyframing if there is a lot of movement in the shot.
The IBK/PIK nodes are very powerful nodes but can sometimes be an overkill for basic keying jobs and they too can be used for multipass keying. The PIK Color node that is the complement to the PIK node performs some clean matte generation which is sometimes not very good on certain green screen saturation. It is always good practice to use multiple keyers when needed to protect the integrity of your alpha channels.