The beginner artist is impatient. They will start drawing a page without fully considering how all elements interact with each other, thus creating a page that looks unbalanced or even chaotic.
A full manga page is fairly large, a lot of stuff can be going on there, and it's not easy to look at it as one homogeneous entity when drawing it.
That's why a pro will begin by quickly drafting very small, rough thumb sketches for their pages.
This way, it's much easier to view the page layout as a whole. Because thumb sketches are so small, there's no chance of losing yourself in the details. You can fully focus on the big picture.
