When "Any Knucklehead" Can't Do It
You don't need special training to haul an armful of 2x4's across the jobsite, and most anyone can manage a hammer and nails, help to pull a rope to lift a wall, or attach hurricane ties to the joints. A lot of the skills that are required on a jobsite are easy enough to pick up as you go. Even the stuff that seems impossible to the greenest gofer on the first day of the job usually sinks in with a little on-site experience.
But, that doesn't really describe every job on the site, does it? No experienced foreman hands the new guy a box of copper and says "Go install the wiring right quick." Are there some tasks on the site that demand less training and expertise than their respective unions would like to think? Well, that's a debate for another day. But it's hard to deny that there are some jobs that not just any knucklehead can get done.
Masonry
Laying tiles and brick isn't the most complicated job on the site, but quality masonry is a little more nuanced than slinging cement and gluing bricks together. Stonemasonry, for instance, involves the use of actual stone, and results in a wall that can last for decades with little to no maintenance. Basic brick-laying is something that you can learn in a weekend of apprenticing, but serious stonemasonry demands real training and experience.
Plumbing
Sticking two pipes together isn't rocket science. You might or might not be able to hook an entire office building up without help, but most anyone who's worked on a building project can figure out most of the basic tasks involved with plumbing. A big part of what a professional plumber brings to the table is an intimate knowledge of building regulations, safety standards and other laws and guidelines regulating the field. A professional not only ensures that the pipes work just fine, but that you don't get hit with an order to tear those pipes out of the walls and start over because you failed to file the proper paperwork.
Drywall
You don't need special licenses or permits to lay down some drywall, but it really isn't something that any gofer hopping off the back of a truck can handle. It's not so much that drywall is difficult to do, but that there's an artistry to it if you don't want to wind up with big globs of plaster sticking out under the wallpaper. On that note, painting is a task that's easy to do, and not so easy to do right.
Over time, you'll learn where each worker's strengths and weaknesses lie. Not every task requires a special license, but some do demand a little more experience than others.