In Product:
For the most part, hero illustrations have no place in product. A product should be functional. It’s not about “making you want to come back every day” it’s about being so seamless to use that you look up and you’ve been using it every day for a year. Hero illustrations can get in the way of that. There are a few notable exceptions. A welcome screen should be pretty damn happy to see you, so I s’pose — go nuts. In larger products you might also find them in on an up-sell page, which is basically a brochure page contained within the product, soooooo see: Marketing.

In Marketing:
Yeah I mean you knew where this was going. Hero illustrations are a marketing tool. The marketing hero illustration must be a captivating introduction. A massive point needs to be made. You need to be left both understanding what is happening, and wanting to know more.