Why would she need my current salary, rather than my salary
Ideally she wants both.
When people are asked their "expectations" they sometimes respond with their "wishes" instead. Everyone wishes to get as much as they can possibly get. Most candidates are realistically expecting to actually settle for more than they are currently making, but less than a huge increase.
Your current salary tends to indicate what you were willing to settle for in your last position. It also indicates what at least one other employer was willing to give you.
And your current salary can be verified independently (while your expectations cannot).
Many employers don't want to give you a huge increase from your current salary, even if that's what you would wish for.
Knowing your current salary tends to indicate to the potential employer if you will fit into the salary structure at their company, or if you won't.
My strategy has always been to avoid giving my current salary when I can. And when I can't avoid it, I usually give my current salary in addition to my expectations. I might say something like "I'm currently making $X with a Y% bonus, but I'm looking for a bit more in my next position" (if that's actually the case). Or I might say "I'm currently making $X, but I'm open to all salary offers" (if I might be willing to take less than I'm currently making).