We all get the fear.
I was terrified mine was growing whilst I waited for surgery. However with neoadjunctive chemo you get 2 Her2+ targeted treatments...something you are not offerered after surgery unless it's spread. So I believe that's probably why they are doing it. To lessen all risks prior cutting in to reduce spread and also it will help them determine your tumors response to treatment so will know how good the chemo mop up is. Your herceptin will continue well after chemo so it will still be working wonders if there are strays.
I hope that makes sense.