You might question the necessity of an explanation for something that seems so obvious at first. Having observed students, I can attest that it is one of the first stumbling blocks and sometimes the habits are hard to break.
But how hard can it be, drawing and painting with a pencil or brush is simply a mater of moving them back and forth, right?
If you were painting the house, then yes, a back and forth motion gets you the most coverage. With artwork however, you’re looking for something different, irregularity and expression.