This medley isolates and annihilates the delts. It hits them from multiple angles, which is important for full development. Four different variations are used, all done back-to-back without rest to increase time under tension. Perform 3 sets, 90 seconds in between each set.
Sure, if you add 100 pounds to all your big lifts you'll definitely have a lot more muscle mass. But it's not necessarily the fastest route, especially for muscles like the delts.
The shoulders are fragile. The shoulder joint is the least stable joint in the body and very complex. There are more than ten muscles that directly influence the way the shoulder moves, and that joint can move in pretty much every direction. So it's very easy to become unbalanced and develop nagging and persistent shoulder aches, if not severe injuries. That's the problem with relying only on the big basics: You stand a much better chance of developing strength imbalances that will impair your performance and ultimately your size.
Furthermore, many strong muscles assist the deltoids. As such, it can become hard to stimulate the delts fully with only big lifts. Sure, the delts receive some loading, but other muscles can contribute disproportionately more and will grow faster. As they grow faster, the shoulders become proportionally weaker and harder to involve – the body will use the stronger muscles to do the job. Injury is then more likely.
This is why I like to use time under tension as well as multiple angles to develop the shoulders. In fact, I want to use as many different angles as possible when working the delts directly. This will not only make the shoulder more functional and injury-proof, but it'll also maximize size.