What are those dark spots when using Subsurface modifier

Hi, i was modelling a car and in some areas on the car, there are these dark spots:

i know its due to subsurf modifier but what is their purpose?

In edit mode you edit original mesh and its intersecting with subdivided mesh. So you see this overlap. You can avoid that by pressing triangle on subdiv modifier, but it gets wonky to edit, but you see more final result

alright thx