Logos, product concepts and mascot or character (games, animations, etc) are all things Illustrators can help you with.
Scamps, sketches or high-spec drawings are sometimes the only way to express an idea or tell a story. For example, it's hard to imagine a property developer selling their development without first creating an illustration showing how it impacts the local area.
Illustration is typically something a skill that's needed for a very specific output. In other words, it's an ad-hoc requirement.
The best way to hire an illustrator on demand is using remote freelancers. The sorts of illustration project you can outsource are: