Fitness

It's good to have a routine that targets a rounded balance of muscles, particularly stabilising ones to avoid injury.

It's actually counterproductive to do leg day vs arm day in theory, as the more tired you get the less effective other exercises are going to be and the higher the risk of injury.

In general though because we have lives to plan around, and often cardio or like a job which heavily relies on legs it kinda sucks to be doing that on tired legs all the time.

So while biologically less productive than splitting up different muscle groups through the whole body in practical terms it's often easier to practice.