Question

In order to accomplish the same goal, a person can click and hold down the mouse button and drag to the right, or hold Shift and use the right arrow key. What is the goal?

Answer 1

The goal is selecting text in the document.
You can try in on your own - just open a Word (or whatever program you use) document and click on a word, and do what you wrote above and you will see that more and more parts of a word or more words will be selected the longer you hold Shift and move the right arrow key.