Scoobyclub's answer isn't entirely correct. The things you learn here are not about drawing in a particular style - it's about ingraining certain habits and rewiring your brain to think in a certain manner. When it comes to drawing lines, you'll notice that if you were to try and draw in a "sketchy" fashion, you'd probably end up with a big mess. Reason being, while the artist you linked to there may be putting down a lot of fluid linework on the page, they're constantly thinking about which mark they're going to put down on the page.

Even if it's just for a split second, their brain considers the nature of the mark they want to draw. That is the habit learning to use the ghosting method here will train in you. If you look at the first few videos in Lesson 0, the video itself is just of me drawing something while I explain what Drawabox is about. You'll see that I don't employ the techniques covered in the course, but that everything I do is influenced by the processes we learn here.

The simple answer is that you are in the wrong place to learn sketching. It's a different skill and not the purpose of these exercises. Following the rules for each lesson very, very closely is a requirement here.

I think the point is that you need to learn HOW to draw lines smoothly in a single stroke. You don't HAVE to do it always if you prefer to do "fluffier" lines for style or something - but you have that tool if you need it!

The problem for a lot of artists is that they ONLY know how to do the scratchy, interrupted lines, and there are times when smooth strokes just look better.

Uncomfy already covered the Draw a Box side of things, but I do think it's important for people to realize why sketches can look so good compared to completed lineart.

When you look at those examples, there are many individual lines involved. That part is obvious, but what is less obvious if how your subconscious decides to interpret those lines. The brain is great at picking out the best parts of each line, or the average of all the lines, to combine them all and make them seem better. In other words, your brain fills in the gaps and gives you outcome that looks like most "right"

This becomes an issue when we go to do lineart because we have to choose which line(s) is correct....and we often choose wrong. Like Uncomfy said, artists have to think a lot about the nature of each mark they draw. If you aren't practiced at considering what you're drawing, and are only thinking that you need to trace your sketch, then you'll often find yourself led astray.