I don’t think that was her plan, at least not initially. It’s been a while since I have seen this episode but I remember correctly she said that she came to kill Angel and to have this object that allow to see the senior partners ( it’s sort of gloves ?). I don’t think having sex with Angel was her initial plan. But of course she changed her mind once she has noticed that Angel wasn’t very well.It was also a stupid plan, one that hadn't been completely thought through, and I'd never thought of Darla as being stupid before this. As a prostituted woman in her mortal life, she of all people should have known that sex alone can never be enough to bring about 'true happiness' and the enormous change she wished for Angel.