Error messages communicate that something has gone wrong to users. Helpful error messages go one step further and let them know what has gone wrong and how to fix it. Imagine that a user has entered the wrong password to their account. In the absence of a helpful error message, they are likely to develop resentment from having to figure out why they are unable to log in. Letting them know that their password is incorrect or that the input field is case-sensitive can allow them to rectify the situation easily and get past the error.