WordPress shortcodes are a powerful feature that theme and plugin developers use to give users advanced tools at their disposal, but they continually confuse people. They are infinitely easier than writing actual HTML and CSS but can still freak out the typical user.
Why are shortcodes useful?
Using only the WordPress editor, one’s options for laying out a page or post would be limited to one block of text, perhaps interspersed with an image, video or maybe a photo gallery. What if you want to create a more visually interesting layout? Such as splitting your content area into columns, for example? Or including buttons, highlights and other fancy visual styles?
To write that kind of HTML and CSS would be quite tedious and beyond the abilities of most users. It would also create a lot of messy code in your WordPress editor, muddying the lines between content (which is primarily what you should be editing in WordPress) and code.