Some artists sculpt their designs instead of drawing them, other go directly to edit mode. I like this phrase from this blog post:
“The key thing when you making a concept in ZBrush is not to worry about tools, but be familiar with them because your mind must be free from anything else but the design of the character.”
She uses ZBrush because she trust her sculpt skills more.
Concepts are made because in our mind things look amazing but blurry, sometimes in constant change. It’s okay if you are not good at drawing, you are locking your design, to make a better thing out of it.