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We all know that sunflower's flowers move so that they face the sun. But the photosynthesis happens in leaves, not in flowers. Why is then so important for a sunflower to move its flower, if its leaves more or less stay put?

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    Leaves of most plants move also with the sun (phototropism), this is caused by growth hormones that get relocated by light (so the side of the plant facing the sun is growing less, and hence faces the sun).
    – benn
    Commented May 15, 2018 at 8:06
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    The flowers are surrounded in some hefty greenery. I imagine some photosynthesis occurs there, whether or not it's responsible. Also note that a sunflower is said to be comprised of many flowers within the 'flower'.
    – Brōtsyorfuzthrāx
    Commented May 16, 2018 at 2:09

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Sunflowers are tall, so contrary to most of the compositae (Asteraceae), the flowers appear vertically (and not horizontally). In this manner bees will see the flowers without the need to fly very high.

But this means the flower is seen just in one direction. By moving towards the sun, the sunflower will be never in shadow, and it will be very visible to bees. Bees' eyes seem not to be so good.

Note: when it makes seeds, the flower will turn down, so that it will allow just some small birds to eat the seeds (large birds will possibly break the flower, and eat too much, so not spreading the seeds around).

So sunflowers are just voluble flowers: they want to be seen. Note: the petals are just for display, the real fertile flowers are in the inside circle. In general flowers are a very expensive gift to insects (petals, nectar, pollen) just to try to have pollination between the same species.

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    Do we know (scientifically) that the reason is visibility? Is there no process within the flower that benefits from sunlight (ie does some biological process benefit from the warmth of direct sunlight vs being in a shadow?) Can any work be referenced that supports this answer?
    – dwizum
    Commented May 15, 2018 at 13:59
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    @dwizum: Flowers are all about visibility. For references, you should ask to biology.SE. Here we keep things practical, and understandable (and maybe not 100% correct). Just finished a course about pollination syndrome, so I tried to apply what I learn (how to guess pollinators and continents just by looking the flower).
    – Giacomo Catenazzi
    Commented May 15, 2018 at 15:45
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    Whether or not bees' eyes are good, I've definitely noticed more of them in the sun than in the shade. Same for wasps. Aphids, whiteflies, and leafhoppers seem to like the shade, however.
    – Brōtsyorfuzthrāx
    Commented May 16, 2018 at 2:05

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