Here's a good explanation I found: Useful means having a practical or beneficial application. Helpful means providing aid. So, useful is more often used for machines or tools, while helpful is more often used for people.

The two words are very similar (and people actually use them interchangeably sometimes), but it really comes down to whether you're talking about a person (helpful) or an object (useful). A person "helps" other people, so a person is "helpful." A machine or object, such as a computer, is "useful" because you "use" it.

Hey Julia, Useful: "able to be used for a practical purpose or in several ways." (So we can say that you can't use it with people unless you're using them) useful advice, a useful book Use helpful with people or things, no difference. a helpful person , helpful advice

Helpful means when someone can help you with something. Useful refers to an object that you can use, that will be helpful with, to complete tasks.