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 the email in my inbox.
General rules, then, to follow:
If you’re trying to get people to buy, share, subscribe to, or recommend something, focus on usefulness. Focus on what people are trying to do and show them something (a product, a method, a service) they can use to do it. Give them scissors if they are trying to cut.
If, on the other hand, you’re trying to get people to improve or develop what they already are doing or want to do, focus on helpfulness. Provide them with tips or guidelines for doing something better or getting better results – things that have worked for you or other people. Give them the ruler to help them cut straighter. If a person thinks something may be helpful in what they already want to do, they will definitely consider it.
No one category (useful or helpful) is better than the other. But it’s important to know the difference so you understand how to get the results you want.