-다면 sets up a supposition and 하다 supplies the doing. The commonest use here is encouragement: 하고 싶다면 하게 두세요 'if they want to do it, let them', 있다면 해라 'if there's something, do it'. 그렇다면 is the frozen form meaning 'in that case'. -다면 treats the condition as hypothetical, where plain -(으)면 suits conditions the speaker expects to actually happen.