Useful lets you get the job done; helpful lets you get the job done better. Scissors, for example, are useful. A ruler to help you cut straight is helpful.
Most people (including myself at times) have a hard time understanding this distinction. They give a person a ruler when she needs scissors, or they give scissors when a person already has a pair.
Example: I often get emails from Barnes & Noble with 15% discounts on books. These emails are supposed to be incentives for me to come to the store or check out books online.
But if I’m not currently looking for books, then the emails just become a waste of space in my inbox. Here the discounts would be helpful if I needed to buy a book, but they aren’t particularly useful to me because I don’t need a book. And so I usually delete the emails.
Contrast this with Amazon, who rarely sends me discounts, but who does email me recommendations for books based on previous books I’ve bought. This, to me, is useful because if I bought a book on email marketing, for example, and didn’t like it, I could check their list of recommendations and maybe I’ll find another book. As a result, I end up keeping a lot of their emails in my inbox.
What people will pay for is usually something that is useful. In the information age, we are bombarded with “helpful” tips on how to do things. But what we are really looking for is something that will teach us how to get the job done – something new, something we’ve never done before.
It’s nice to be helpful, but if you want users – people who are actually going to use your products to create value for themselves – then you have to be useful. Let them do whatever job they need done.