Full body training vs Split muscle training

Why full body for a beginner?
The muscles are not yet conditioned to extensive and demanding workouts, thus they will fatigue relatively quickly. On the flip side they will also recover relatively quickly.
Also because of this, it will not take a tremendous amount of stress on the muscles to elicit a response that results in greater strength and some muscle growth.
There will come a time when you will hit diminishing returns on full body and may consider a split routine. That will hit after about 18 to 24 months, depending on the person's physiology and how they train.

why compound moves?
In a word "efficiency"
What are they?
Lifts that use multiple joints. As a result that lift works multiple muscles. You get more work and results from a compound move than from an isolation move. This is a better use of your time training. If you are already a well trained, high performing athlete, then isolation moves are great for addressing specific weaknesses. For the beginner, compound moves deliver far more results for the training time put in than isolation moves.

For example:
Lets look at a couple of leg exercises:
The leg extension - it works the quadriceps (front of the thigh). It is a great move for isolating the quadriceps, which makes it well suited for the experienced lifter wanting to hammer the quads at the end of there leg routine. However it is very ill suited for overall leg development of a new lifter.

Front Squat or Back Squat (whichever you are more comfortable with) - The squat will work the glutes (butt), hamstrings (back of thighs), and quadriceps (front of thighs), and to a lesser extent your abdomen and back. 3 sets of these work many more muscles than leg extensions. As a result you burn more calories, put more muscles into the recovery process and as a result build more strength.



It's a simple economics question at that point. What gets you a greater return on your gym time.