If a girl received flowers (like a bouquet, not just a single flower) while I was working with them, I'd immediately wonder what the guy did wrong (terrible first thought, but it's true).
When my SO and I first started dating, he left a super pretty flower on the windshield of my car while I was working. The only person who noticed was me, after driving halfway down the street without noticing it, haha.
I'd assume it was her birthday. That's the only time I've seen it.
My husband has never sent me a bouquet of flowers, like others here, I just see money wasted. When we dated, I'd sometimes get one or two cut roses, but even that was rare. My parents have sent me flowers a few times, and I really appreciate the thought, but I hate the waste of money, especially on delivered flowers! (I THINK I've finally talked my mother out of sending me cards for my birthday and anniversary. Greeting cards are like $3-4 each now. That's ridiculous.)
All this reminded me of an Aunt I had. We got $15 from her each Christmas, but we always visited her on New Years. If we'd let her hand us the check on New Years, instead of mail it down, she would add 25 cents to it. I guess that was lazy, since it didn't save her money, but I always wanted my extra 25 cents, a lot of my cousins wanted the money on Christmas (so they had day after Christmas shopping money).