Some plants the flowers are very pretty, but the whole plant isn’t practical to be kept indoors (e.g. roses), and they don’t necessarily flower all year round. If someone has a favourite flower or favourite colour of flower, it makes them feel special when someone thinks to buy them for them (they get a flower they love to look at for the next week, and they know that someone cares about them enough to buy them flowers, and keep the knowledge of which like).