The potential subjunctive is one way to use the present subjunctive in Latin: looking (slightly) into the future and speculating on what might happen. It's the difference between "she wants" (vult, pres. indic.), "she will want" (volet, fut. indic.), and "she might want, maybe she wants" (velit, pres. subj., interpreted as potential).
All the options (velim, velis, velit) are possible. “I, a girl, would like a pastry”; "You, a girl, would like a pastry"; "She, a girl, would like a pastry” ( = "The girl would like a pastry”).