Flowers are an excellent gift.
Everyone knows what they are, they have a social brand recognition that is only matched by the little black box of an engagement ring, as soon as you see it, you know what it is.
Flowers are bold, there is no hiding them. If you walk in to your girlfriends office and drop off a nice watch for her, when you leave all her co workers will ask “who was that guy”. If you walk in with a big bunch of flowers, after you leave, all her co workers will ask “what does the card say”.
You could say flowers are unoriginal and a default. That it is kind of like saying “I could not think of anything else so I got flowers”, but I like to think of it more as saying “I got you flowers because nothing else would do”.
They are also a great tool for expressing your self, either in an original way, or by following social preset standards. Everyone knows what one red rose means, we all know why it is different to 20 pink roses.
You could buy her 19 real red roses, then slip in a fake plastic one in the middle, and send it to her with a note reading “I will stop loving you the day the last rose dies”, and make a bold yet corny statement, or just pick up a big bunch of colorful ones, just to score some brownie points.
I enjoy getting flowers for my girlfriend because of the obvious message that goes with it, and because it brightens up her day, and it makes her feel special. This is also the same reason why I like getting flowers.