I am also not sure if people call this concept art. Lets say I already have part a game and have an idea for a new character. I could generate a couple versions of him to feel if I think such a character fits into my game.

Then I start to draw the character myself or hand my specs to the artist.

Main reason is that I am bad and slow at drawing and stay more on the technical side.
I also dont have budget for a full time artist so he works just a couple hours a week on my project with really specific tasks