On his ack/nak blog, Bob Corrigan wrote about how Gmail has at long last added a delete button so that you can delete e-mails you no longer want to keep. I've implied in the past that Gmail's lack of a delete button was actually the result of a great requirements insight.
However, so many users have demanded the delete button that Google finally added it to Gmail. It's an interesting case, though, because it's likely that the vast majority of users who clamored for it have no concrete and practical reason for it. For example, Corrigan doesn't claim that the unnecessary e-mail affects his ability to find wanted e-mails or have caused him to run out of storage space. Instead, he writes vaguely that he doesn't want unnecessary e-mails "in his stack" and even that he may be a little compulsive about it.
I do believe there are some practical reasons for the delete button, such as keeping e-mail from exes out of sight from current significant others who may have access to your e-mail. Nonetheless, in the end, Google had to accept that largely emotional (perhaps even irrational) factors affected the usability of their e-mail product.