In reference to a digital product like a website or system, utility refers to if the product provides the features that you want. Regarding utility, ask yourself "Did the site actually have what I was looking for? Usability refers to how easy the interface is to use. Regarding usability, ask yourself "How hard was it to find what I was looking for?"
Even further, usefulness = usability and utility or whether the product was easy to use and provided what I needed. Therefore, in order for your digital product to be useful it must provide utility and be usable.
This knowledge article provides a short video and information on the importance of each attribute.