You should discuss...the feature and pop the why stack max 5 times (ask why recursively) until you end up with one of the following business values:
- Protect revenue
- Increase revenue
- Manage cost
If you’re about to implement a feature that doesn’t support one of those values, chances are you’re about to implement a non-valuable feature. Consider tossing it altogether or pushing it down in your backlog.
(If you follow the link, you can also read the transcript of an IRC session in which Aslak helps someone pop the "why?" stack for a "login" feature.)