Most of us started bodybuilding/weighlifting, doing the old-school training three times a week (Monday, Wednesday, and Friday), as a beginner routine.
I did that for about a year, before I split my routines up, as recommended by the late Mike Mentzer in an old Muscle & Fitness magazine from 1989.
It wasn't until 1994, when I simplified my splits to where it was upper body on Monday and Thursdsay and legs on Tuesday and Friday.
However, IFBB Hall of Famer Leroy Colbert (the man credited as the first to build 21" arms) says that people should STILL train their whole body and rest the next day, citing split routines as more suited for steroid users.
I don't necessarily agree with him on this one; but, I thought it an interesting take.