It is a "minimalist" routine that optimizes recovery.
Squat, Bench and Rows covers all the body's main muscle groups.
Deadlift, Press, and Chins covers all the body's main muscle groups.
Go to the gym, work hard on three BIG lifts, do enough to stimulate growth, go home and rest and grow. This split allows you to gain on the minimum amount but still allows for the use of some different lifts. Squat and Deads hit pretty much the same muscles, Bench and Press hit pretty much the same muscles. Rows and Chins hit pretty much the same muscles. See the pattern? No sense doing both Squats and Deads in the same session if they are doing pretty much the same thing. Also, six BIG lifts seems about right for the average person to push hard and make consistent gains. Try to add too much too soon and something will stall and not make progress.