here are only a few good reasons to use illustration instead of photography for picturing a product that already exists. With the item available, it is hard to imagine another more efficient method to create its image than that which can be achieved with a camera and some retouching. However, in the preproduction stage, before there is a finished product to photograph, illustration is the best answer.
Another situation when illustrations are used is when art direction deems it to be the right solution. Either for want of a particular style or because of an established branding strategy, the judgement has been made that an illustrative look will outperform photography. Since these decisions aren't usually made frivously, it is always interesting to encounter such an approach.
Illustration is also used to accomplish a transparent or cut-away view. While photography can also work, each individual case can only be decided on its own unique circumstances.
How to show both inner and outer beauty.
This carryall has more pockets and compartments inside than out. For use in a direct mail piece, the challenge was to depict all of its functionality. The solution was to create a transparent bag. While this could have been achieved photographically, the time and expense involved to do it well would have been prohibitive. For this specific project, it made better sense to use illustration