Playing with four sticks can produce a flam effect. And with two sticks on one hand, it also enables us to play with two different instruments or sound sources. Just like what Steve Gadd did in the video, he played his ride cymbal with one stick playing the bell and the other playing the ride area.
A few famous drummers also do that, like Carmine Appice and Ginger Baker, I think. Louie Bellson is also famous for his four-stick technique on his legendary drum solo "Skin Deep". And John Bonham also did that on the song "Four Sticks", from Led Zeppelin's IV album (hence, the song title).
