I think they are largely interchangeable. I'm trying to come up with sentences in which one works, but not the other, and not doing very well. I think "useful" is a little more broad, but I can't explain why.
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useful is usually used to refer to something is providing benefits or is providing a advantage. while on the other hand helpful is something we say that is giving aid on something, something like a "tool".
They are synonymous; however, when describing people, or people's actions, it is much more common to use the word "helpful."
Common:
Tom is a very helpful person.
(to use useful in this context would be awkward/rude to some, although some would use it)
Jane was h