Why is it equal to $H^1(X,T_{X}(-p_1, -p_2, ..., -p_k))$? And what is the motivation for studying this? (I know the proof for unpointed curves, which can be found in a few sources: Hartshorne's deformation theory, Arbarello, Cornalba, Griffiths' Geometry of Algebraic Curves II etc.)