A beginner in golf should hit from the closest tee box because it is almost always the easiest path to the hole. Because most beginners don't have great technique yet, it is more important for them to focus on their aim and consistency than it is to focus on their power.
When beginners try to hit a ball too hard, they tend to "top" the ball - which results in the ball barely moving - or they tend to "slice" or "hook" the ball, which is when the ball veers off into an unintended direction. Because the beginner tee boxes are so close to the hole, they encourage players to focus more on their aim than their power.
Hitting from a closer tee box also makes it easier to correct mistakes. If a golfer's first shot is bad, it is easier to make a better shot on the next one if they began closer to the hole.
Once a beginner has the proper technique down, they can begin to tee off of the farther away tee boxes, and try to hit with more power. Some golfers have a lot of power naturally, so they can shoot from a farther box sooner. Over time, a golfer will get both stronger and more accurate, and eventually they can move to the championship tees, which are farthest away and involve the trickiest paths to the hole.