An illustration becomes a part of the business' brand image when it's used on it's media, like it's logo, brochure and business cards. Illustrations are used in advertising because they convey an instantly recognizable message. You can set the mood, tone, and style in one glance.
Illustrations, sketches, drawings, photographs and paintings: Any image representing something is another word for illustration: sketches, drawings, photographs, paintings. The only substantial difference between them is the format it's displayed on (web, paper, car wrap). And in today's world where artists are more technocratic, the format is less critical. But, at the end of the day, the aim of an illustration is still the same: to send a message.
Where you'll find illustrations: You'll find illustrations everywhere: publishing, transaction printing, images on blogs, business cards, internet memes .. you name it, it likely has an illustration associated with it.
Illustrations and graphic design: In graphic design, the illustrations should be created in a format that's easily outputted to a printer at the correct resolution. If the format isn't correct, it usually requires that the artwork is redesigned from scratch to ensure the printer prints it out correctly.
Digital Artists designing for business often use vector or raster image files.
Digital artists can create files in either Raster or Vector images file types. The final output dictates which file type is used. This is why most corporate illustrations are created with Vector: vector lacks pixels. A vector graphic design is entirely created from multiple points on a graph. These kinds of illustrations are freely scaled up or down without loss any of detail. The benefit here is that the company isn't bound to single-use designs, which mean they can reuse their designs over many different mediums and channels without having to recreate the image each and every time.
A short comment on custom illustrations: One fantastic way to visually define your business is through custom a illustration. By developing a unique style and voice through illustrations, you bring out what's unique about your company. And this will help you attract the kind of customers who are interested in your product.