Not only girls & flowers, bro. Everyone will be delighted when they receive a surprise gift be it flowers, chocolates, wine or anything that they like. It makes them feel special.
Generally, girls do like flowers and when they receive flowers out of surprise, they feel that they are special and they like the attention on them.
I have a colleague who received a nice piece of cake from a rather popular cake shop on her workstation yesterday. Although she is on a diet and do not really take cakes now, she was delighted. It's the attention & affirmation that matters not the gift itself.
I know one girl who doesn't like flowers, but then she is the geekiest girl i know.
Also, not all girls always "feel special" on gifts - If your gift is not something that matches the relationship between you two, (to take an extreme example, giving a diamond ring to a girl you have talked only twice) you might end up looking like an oddball/idiot/desperate or even worse, like trying to buy her off with gifts.
Hmm.... I'm not sure but when I asked my lil' sister this question and asked her what would be her reaction if a guy gives a girl a bouquet consisting of Carnations, Tulips and Lilies dotted with Babies Breath...