To save energy. The answer is based on the fundamental principle of free energy, that everything in nature seeks the lowest possible energy state.
In his study which appears in the ACS' journal Langmuir, Martin E. R. Shanahan observes that drops of water have a preference for exactly where they collect on leaves as their surfaces cool in the morning and afternoon.
Those droplets, which condense from moisture in the air, collect randomly across the surfaces of flat leaves. However, dew drops tend to accumulate at the tips of spindly leaves, even if that means defying gravity by moving upwards.
He explains that an inherent "unwillingness" or "lack of necessity" of water drops to move on a dry surface governs their positioning on flat leaves, causing them to stay where they form. Dew's tendency to head to the end of finely pointed leaves, however, sent Shanahan looking for a different explanation.
Shanahan modeled two types of dew drops on a theoretical (simplified) cone-shaped leaf: a thin, cylindrical sheath of water and a spherical drop centered on the cone's axis. In both cases, he found that the drop lowered its energy by moving toward the point of the leaf.