What is the advantage to browning steak first, before slicing/cooking it?

I'm not a good cook, but will be making Rachael Ray's philly cheese steaks for my hubby for Father's Day. Would it be "bad" if I don't brown the flank steak first before I slice it and cook it? I'd rather have the butcher slice it up since I know I won't do a very good job of slicing it myself... Thanks!