Flowers remind us of Mother Nature at her most lovely, which is always helpful when dealing with romance. Plants are lovely, but not nearly as romantic. Flowers symbolize new life. It's no wonder suitors give their loved ones flowers to show the blossoming of their love. Interestingly, flowers also symbolize purity and virginity.
Specific flowers have specific meanings, as well.
Though it's open to interpretation, here's a consensus of what certain flowers mean. Trust me, anything that works on the designated holiday works even better on a random day and is scads cheaper. The point isn't the lavishness, but the thoughtfulness. Guys like getting flowers too, just ease off the roses. The message is still romantic and thoughtful and sexy.