Why does the SATA power connector have so many pins?
The connector shown in the image is a 15-pin SATA connector. Pin description:



The connector can have 5 wires. And this particular connector shown in the question is missing the 3.3 V (orange) wire.

The new SATA power connector contains many more pins for several reasons:


3.3 V is supplied along with the traditional 5 V and 12 V supplies. To reduce impedance and increase current capability, each voltage is supplied by three pins in parallel, though one pin in each group is intended for precharging.
Five parallel pins provide a low-impedance ground connection.
Two ground pins, and one pin for each supplied voltage, support hot-plug precharging. Ground pins 4 and 12 in a hot-swap cable are the longest, so they make contact first when the connectors are mated. Drive power connector pins 3, 7, and 13 are longer than the others, so they make contact next. The drive uses them to charge its internal bypass capacitors through current-limiting resistances. Finally, the remaining power pins make contact, bypassing the resistances and providing a low-impedance source of each voltage.

This two-step mating process avoids glitches to other loads and possible arcing or erosion of the SATA power connector contacts.
Pin 11 can function for staggered spinup, activity indication, both, or nothing.


Source: wikipedia article on Serial ATA.