Pins have equivalent inductance, and when you're switching, let's see, about 30A in 10 nanoseconds, every nanohenry counts.
Each pin on a D2PAK sized device has about 4nH inductance. (For a regular D2PAK, that's all you need to know. For this one, the pins are connected in parallel, so the inductances also act in parallel. You can ignore coupling between them, for a wave-of-the-hands analysis.)
The Kelvin source connection, for gate drive, is very nice as well.
Homework 1: given the numbers I just wrote, how much voltage error, or overshoot, do you expect? Where does that error appear in the circuit? Why would it be important? How much difference is there, using one pin versus all in parallel?
Homework 2: draw out the equivalent circuit of a transistor, including its pin/lead inductances, stray trace/wiring/PCB inductance, and so on for everything else in the power switching circuit. Show us your schematic, and reflect on its meaning.